Add avatar SDK support

[BastiaanOlij]

The oculus SDK has a whole sub-SDK for avatar support:
https://developer.oculus.com/documentation/avatarsdk/latest/concepts/avatars-gsg-intro/

Part of the oculus system is for the user to setup and customise their avatar that they wish to use in game. Being able to interact with this SDK and display the render models inside of a Godot game would be a nice touch.
This SDK also gives access to things like the touch controller render models and hand gesture models.

This would add a platform dependent layer to the interface so it has to be optional.

[codemonkey-uk]

The SDK link is the ticket is out of date. The native avatars SDK link is now here:

https://developer.oculus.com/documentation/native/android/as-avatars-sdk-intro/