Add support for custom sample frame rate

[pixel-dust-dev]

What

Added support for choosing a custom sample rate for animations from the Anim Importer Editor Window.
The time differences between keyframes will be affected in the animation clip.
The animation clip sample frame rate value will be updated.

Use Cases

Stylistic Choice
More consistency for an animator who is intentionally aiming for a stylistic choice based on a set FPS.
Redundant in between frames are not needed and are simply waste.

When using a frame based move editor, designers can spend less time scrubbing through empty animation frames when placing hitbox events, sound events and the like.

Less frames can be used for background elements to save processing unneeded frames.

Considerations

When importing at a lower frame rate the timings will lose precision.
The lower the sample frame rate, the more likely that Aseprite playback and Unity playback will diverge.

Tests

60FPS
Length: 0.8 seconds
48 Frames

30FPS
Length: 0.8 seconds
24 Frames

12FPS
Length: 0.833 seconds
10 Frames