Add unstable_getViewTransitionInstance to UIManagerBinding (#55940)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55940

Adds `unstable_getViewTransitionInstance` API to UIManagerBinding and FabricUIManager, allowing JavaScript to retrieve position and dimension information for view transition pseudo-elements.

This enables us to obtain pseudo element created on the runtime and manipulate them. React allows imperative animation commands like below:
```
<ViewTransition
   onEnter={(instance)=>{
       instance.new.animate(...);
    }}
>
  // ...
</ViewTransition>
```
on RN we could support something similar, e.g. letting user animate the new instance with Animated at VT event handlers, this could potentially guarantee smooth animation, since when onEnter is invoked, new shadow tree is committed but native view should not be mounted yet.

## Changelog:

[General] [Added] - Add unstable_getViewTransitionInstance to UIManagerBinding

Reviewed By: sammy-SC

Differential Revision: D94956110

fbshipit-source-id: 1d243b308033ce927b282e91a271d995fac226cc