This builds on PR #101951 so marking as DRAFT until that one is merged

Refactors as I described in a comment on that PR:

1. Merge the methods from OpenXRCompositionLayerProvider up into OpenXRExtensionWrapper. While using OpenXRCompositionLayerProvider like a C#/Java/PHP/etc interface makes sense, multiple inheritance in C++ can cause weird issues especially in conjunction with Godot's Object and ClassDB system. In the past we had a really hard to debug crash (see Crash on exit when using OpenXRExtensionWrapperExtension #88613) that was the result of this, and I personally think it would be better to avoid the problem entirely by just not using multiple inheritance. (Also, we had some later work that attempted to follow the example of OpenXRCompositionLayerProvider and my review was "let's not do that anymore" because of these issues, but I worry that future contributors may still see OpenXRCompositionLayerProvider and again try to follow its example, so let's just get it out of there)
2. Deprecated OpenXRExtensionWrapperExtension in favor of just using the now-exposed OpenXRExtensionWrapper. We can do this without breaking compatibility by just making the default implementations of the virtual methods on OpenXRExtensionWrapper use the GDExtension implementations from OpenXRExtensionWrapperExtension, and emptying out OpenXRExtensionWrapperExtension but not removing it. Old GDExtensions would still extend OpenXRExtensionWrapperExtension and continue working fine, but newer GDExtensions could directly extend OpenXRExtensionWrapper. This will eliminate the annoying extension-extension we have in OpenXRExtensionWrapperExtension :-)

NOTE: This is 100% backward compatible! I tested it with the "Meta Composition Layers XR Sample" from the asset library, which includes the binaries for version 3.1.2 of the godot_openxr_vendors extension.

(Although, CI is going to complain about missing methods, but they moved up to a parent class which is fine - I'll copy the messages into the ignore file after CI gives them to me :-))