Add binding modifiers to OpenXR action map

[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

The current action map implementation in Godot has a number of missing features that are now available through the OpenXR API. This proposal focuses on adding support for binding modifiers.

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

OpenXRs action map attempts to disjoint the actions a user performs within an XR game from the actual inputs performed on the controllers. This in theory allows OpenXR games to be used on platforms for which it has not been designed by creating the appropriate bindings or introduce peripherals the original game was never envisioned to work with.

The missing component here was the inability to make adjustments on these inputs, be it platform specific thresholds, offsets, etc. Especially making sure poses (spatial locations on controllers) line up has been a big missing feature.

OpenXR added the XR_KHR_binding_modification extension for this. This extension is but a foundation on top of which different modifiers can be defined.

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

This needs further investigation on how we can best integrate this into Godot. At this point this proposal is just a starting point for the work that needs to be done.

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.