feat: Just check Animators for finding avatar root

[kaikoga]

A simpler resolution that closes #68, also ref: #432

I thought this would be breaking change if there were Animator components in the parents of VRCAvatarDescriptor.
However:

• The outer Animator cannot access humanoid animation of inner Animator (on the VRCAvatarDescriptor object,) so the setup makes no sense anyway.
• The outer Animator may be a Resonite avatar in a VRCSDK project, for example.

So I would propose just checking Animators, because I suppose it is safe to assume that an avatar has an outermost Animator at its root object.
It also aligns with the current behavior of RuntimeUtil.FindAvatarRoots(), as it ignores nested candidates.

Drawbacks: Slight cost of RuntimeUtil.IsAvatarRoot(), and RuntimeUtil.FindAvatarInParents() if there are nested Animators. (I think this cost is unavoidable after all.)
I think it can be optimized if we can agree on "outer Animator takes precedence" rule.
(If this cost is unacceptable, so is generic Animator support and avatar root marker component would become mandatory...)

Versus avatar root component type registry: Less overengineering, and less encouraging adding a dummy VRCAvatarDescriptor.

[bdunderscore]

This needs to observe the animator component (context.GetComponent<Animator> or similar) as well as any properties used to determine if the animator is used as an animator root.

It's also preferable to avoid observing the same object multiple times, as each observation registers a callback used to determine if something changed...

[kaikoga]

It looks like essentially same as adding / removing VRCAvatarDescriptor in the middle of avatar hierarchy, so I need a strict (not a heuristic) definition of what is an avatar to continue working on this change.

Also, NDMF Avatar Root (as in #432 ) is not what I can actually start implementing without prior detailed design, because the change will certainly break ResoniteImportHelper and Chillaxins, and breaking non-VRChat support for a long time is not what I want to do.

[kaikoga]

Closing, because we need a different approach.