BabylonReactNative should support initializing custom BabylonNative plugins for npm builds

[chrisfromwork]

Plugin initialization appears to occur for android and iOS in cpp files that are compiled into .so and libs for distribution via npm. This makes it difficult to initialize additional BabylonNative extensions without building directly against the BabylonReactNative repo.

It may be that we decide the best practice is to only allow custom BabylonNative plugin initialization when building projects against the BabylonReactNative repo, but it felt like this issue should be filed for additional discussion.

It may also be that it is indeed possible to initialize external BabylonNative libraries with the current configuration. In this case, we should repurpose this issue for calling out how in documentation.

[chrisfromwork]

most of the initial external plugin work we are doing for BabylonNative will be related to OpenXR usage. This is now unblocked: #163