Add OpenXR Metal support

[BastiaanOlij]

• Production edit: Depends on Add Metal support for macOS (arm64) and iOS godot#88199.

Describe the project you are working on

Core support for OpenXR in Godot.

Describe the problem or limitation you are having in your project

Currently it is not possible to use the Metal back end with OpenXR

Describe the feature / enhancement and how it helps to overcome the problem or limitation

This requires adding support for the OpenXR Metal extension to our OpenXR module, once OpenXR publishes such an extension

Godots metal back-end is still under development and is another prerequisite for this..

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

n/a

If this enhancement will not be used often, can it be worked around with a few lines of script?

This requires a core implementation.

Is there a reason why this should be core and not an add-on in the asset library?

This requires a core implementation.