Add OpenXR DirectX support

[BastiaanOlij]

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 DirectX back end with OpenXR

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

This requires adding support for the XR_KHR_D3D12_enable extension to our OpenXR module.

The main issue to overcome is that current multi view support is broken in our DirectX implementation.

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.