Implement controller model support

[BastiaanOlij]

OpenXR has an extension from Microsoft that when available allows us to load a properly setup controller model for the controller the player is holding in their hands. This is similar to the logic that already exists in the OpenVR plugin for obtaining controller models with the notable difference that OpenVR supplies a simple vertex and index list from which we can create an ArrayMesh while OpenXR provides a fully articulated GLTF (GLB) binary.

See https://www.khronos.org/registry/OpenXR/specs/1.0/html/xrspec.html#XR_MSFT_controller_model for more info.

This is currently blocked by:
godotengine/godot-proposals#1632