Expose the backend via UIManager (#53995)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/53995

### Summary

Exposes the animation backend via UIManager by introducing two new methods: `unstable_setAnimationBackend` and `unstable_getAnimationBackend`. The `NativeAnimatedNodesManagerProvider` is updated to initialize the animation backend using the `useSharedAnimatedBackend` feature flag.

## Changelog:

[GENERAL] [ADDED] - unstable_setAnimationBackend, unstable_getAnimationBackend in UIManager
[GENRAL] [CHANGED] - call unstable_setAnimationBackend in NativeAnimatedNodesManager

Reviewed By: zeyap

Differential Revision: D81137954

fbshipit-source-id: b9a45b1d1e694e28865c27181bcbd578c1b7b9a0