Is there some spell that gives a bonus to Attack Rolls (when using melee or ranged weapons)?
Not so much. There are plenty of buff spells so to speak, but not so many to make hitting easier.
Some spells that you can use:
Edge of the Razor (Grants damage bonus)
Blade of Virulent Flame (Damage bonus)
Hardness of Adamantine (Damage bonus on weapons, Soak bonus on Armor)
Endurance of the Berzerker (Ignore pain and fatigue)
Gift of the Bear's Fortitude (soak bonus on flesh)
Doublet of Impenetrable Silk (soak bonus on clothes)
Hauberk of Supreme Lightness (reduced Load of armor)
There's several more, but these are all buffs. Actually hitting something easier is not easily done in Ars. The closest thing for an effect like that is the Verditius Inner Mystery of Items of Quality. They can add a bonus to weapons to hit (the classic example is a Sword getting +4 to hit humans and animals).
Not much else provides a direct to hit bonus unless it raises the Dexterity of the wielder. Some of the other kinds of magi do have spells and effects that can give someone a buff on their ability to hit. The Vitkir with Rune Magic is a good example.
I was thinking about something that would improve the target's speed and grace for a short duration, giving him a +5 to attack rolls and defense rolls (a Muto Corpus spell?)
There's no real mechanic for that in Hermetic magic. A buff to Quickness would of course increase the the Initiative and Defense of a person, and likewise a Dexterity buff would improve the attack.
But Muto would not be what you'd use, you'd have to go with Creo Corpus, and the power required to make the buff is high for a small amount of benefit.
In the late 4th edition (Loch Leglean Tribunal book) I recall there was a spell that simply gave a straight combat buff to a person, Rego Corpus, but that is no longer available.
What you're likely going to have is an easier time making it so that even a light hit does extra damage (there are 3 buffing spells in that list that add to damage, and I believe all stack, but all would have to Penetrate MR).
But to give a straight +5, there isn't anything, except Rune Magic, which can rather easily give a +5 if engraved into a sword or something.
I was exactly searching spells for a mage that has Hermetic Rune Magic.
I was uncertain if the spell should have been a Muto Corpus, Creo Corpus or Rego Corpus (I discarded Creo Corpus because the effect would give a skill well beyond the normal limit).
On the other hand in City & Guild high quality weapons produced by somewhat better than everyday craftsmen can give bonus to attack, defense, and damage...
No. But are you familiar with the fatigue rules for doubling the weapon ability? Don't discount being Self-Confident (spending two Confidence points at one time can be a game changer. My experience is that combat botches are a bigger problem than needing a bonus, so Cautious with Ability with regards to the primary weapon skill is a good idea.
Oh, and something I've been thinking about for my maga who's primary combat capability comes because of her grogs, and her spells are built around the grogs, involve the MuCo Spell Gift of Vigor so that she absorbs the fatigue of worn out combatants...
My troupe invented the following spell. It's only really good for 'Brawl', but it's something, at least.
Claws of the Cat
Muto, Corpus, Level 4
Requisite: Animal
R: Personal. D: Diameter. T: Individual.
When cast, this spell transforms the caster's hands into a pair of feline claws conferring the following adjustments to the character's 'Fist statistics (-1 Init, +2 Atk, +3 Def, +2 Dam). Naturally, a character thus affected cannot use manufactured weapons.
(Base 2, +1 Diameter, +1 Requisite)
What is the reason for the increase to the Defense stat on the claws? I can see the slower Init, better Damage, and better Attack, but not the Defense adjustment.
I'm not really sure, I just copied the Claw stats from HoH: MC. 
Perhaps, it reflects a range advantage over the closed fist?
In the "Lion of the North - Loch Leglean Tribunal" book (it was third edition) I didn't find anything about this spell.
You don't remember the name of the spell?
(Thanks anyway!)
It was a ReCo spell, I think under Edward of Horsingas if memory serves.
I'll try to remember to look it up tonight if no-one has beat me to it.
I should probably also mention that I don't think it would/should actually work as described.
The closet I can recall to that was in Rival Magic. It was listed in the breakthroughs for the Amazons and the Soqotran sorcerers, but was limited to instilling aiming bonuses into enchanted items and 'storing' die rolls respectively.
And all the spells I know of to raise attributes have been rituals (and permanent). A temporary buff is something I'd check the books again very carefully, as I'm not sure 5th ed guidelines allow for it.
The guidelines for Characterisitic increases do not specify that they are rituals. Keep in mind that all of the ritual spells are high enough to cause warping. So any spell invented based on the guideline has a duration, say Sun, your adding two more magnitudes to the spell. Couple that with the guideline only adding +1 to the Characteristic, your going to see warping build up really fast on frequent recipients of the spell.
Of course there wouldn't be any warping for a R:Personal spell, because it is designed for the caster.
Found it!
It's in line with the idea of the original Rune Magic: +1 to attack for every magnitude.
So a Touch (+1), Diameter (+1), Individual (+0) spell of level 20 would give +2 to attack
Minor thread necro. Not what the OP had in mind but this quote actually got me thinking. While providing a bonus to hit isn't something hermetic magic does easily hitting things with magic is in fact ridiculously easy. Penetration issues aside most spells auto hit without need for a roll. Put an Invisible Sling effect in an enchanted bow and you get a magic weapon that will never ever miss no matter who's using it.
I'd think I'd allow a similar effect for a melee weapon in most sagas. Allowing a sword to always hit an opponent doesn't really seam more powerful then any other auto damage effect.
It would have to penetrate ofcourse, but I think you mentioned?
More interestingly, it would hit automatically, but (presumably) with no carry-over, meaning that the damage of the weapon is based purely on ... the damage of the weapon, surely?
It would be a ReTe based effect. I'd go with a +5.
Quite a sound reasoning with the invisible sling - bow. Never thought about it. Can make for a quite powerful turb with minimal training, and one that will haver problems turning against the magi.
So it would be something like that :
The infallible sword ReTe 18
The sword hits an adversary for normal damage depending on it's type (short +5, long +6, great +9).
The effect has a penetration of 6.
Base 4, +1 Touch, +10 unlimited usages, +3 penetration
I like it ^^