bevy_animation: Abstract away interpolation

[valaphee]

Objective

• For each combination of keyframe type (position, rotation, ..) and interpolation type the interpolation is separately implemented.

Solution

• Implement a trait for all keyframe types which defines the methods required for lerp and cubic spline

---

Should be a non-breaking change as it is only internal

Theoretically lerp could also be abstracted to addition and subtraction and would remove the trait body, but for quats, slerp is used instead of lerp

Weights are also a little bit interesting as I needed a stride for the keyframes array, which is 1 for all other types, but the compiler should optimize it away.

[alice-i-cecile]

I like the direction of this abstraction, but we should consider the interaction with #4482 (which could use some help over the finish line).

[mweatherley]

Closing this as superseded by the curve-based animation restructure (#15434).