Add a SkeletonMask Resource for AnimationTree filtering

[Jesusemora]

some nodes in the AnimationTree have a filter option that allows you to select which bone transforms will be overridden/added to another animation.

but if you use this option, you have to manually select each bone in the skeleton every time.

I propose to replace or extend it with a new SkeletonMask resource. this way you can set up your mask once and then assign it to the filter, allowing you to reuse it multiple times.

This is the skeleton filter right now:

I'm thinking of something like this:

and move the bone selection inside the mask.

[Glitshy]

Just came across this in the context of SekeletonModifier3D, for which this would be great as well,