Support making hybrid apps for Meta headsets

[dsnopek]

This is moved over from godot-sdk-integrations/godot-meta-toolkit#1, and reworked in such a way that it should be easy to support other headset vendors (but this PR only has Meta support). In fact, since godotengine/godot#101387 works, I think we should be able to make this work on Pico as well, but I haven't tried it yet.

UPDATE: I made PR #246 that builds on this one to add support for Pico headsets.

NOTE: This PR only works with Godot v4.4-beta1 and later (and so I've put this on the 4.x milestone).

You can use this export setting to make a hybrid app:

You can choose Start As Immersive or Start As Panel.

And it provides this API for developers (no longer built in GDScript - it's now a singleton implemented in C++):

# Returns true or false for if this is a hybrid app
OpenXRHybridApp.is_hybrid_app()

# Returns an enum with values: HYBRID_MODE_NONE, HYBRID_MODE_IMMERSIVE or HYBRID_MODE_PANEL
OpenXRHybridApp.get_mode()

# Allows switching modes and passing a string as "launch data"
OpenXRHybridApp.switch_mode(OpenXRHybridApp.HYBRID_MODE_PANEL, '{"info": "the info!"}')

# And this function lets you get the "launch data"
OpenXRHybridApp.get_launch_data()

I've included a sample app, with the "famous" rotating turkey. The turkey's current rotation is passed as "launch data" when switching modes, so it'll start from the same rotation that it left off on.

[dsnopek]

Thanks! All review should be addressed in my latest push.

However, moving from an export option to a project setting didn't work when using the "Remote Debug" button (it wouldn't recognize changes to the setting). I just posted PR godotengine/godot#101658 which will fix this.

(FYI, it worked as an export setting, because changing an export setting causes the export options overrides to be updated. However, updating a project setting doesn't. But Godot is updating the export options overrides before attempting an export in all other situations aside from "Remote Debug", so my PR changes that, rather than directly responding to project settings changes.)

[m4gr3d]

It's looking great! Just a few feedback to address and it should be good to go!

[dsnopek]

This is updated for all review, and the last CI run passed (however, it's started a new run now because I just rebased after PR #250 was merged) so this should be ready to go as soon as CI passes again :-)

[m4gr3d]

Looks great!