Add request permissions on start project setting

[m4gr3d]

This PR is a follow-up from a discussion with @dsnopek; it introduces the xr/openxr/extensions/automatically_request_runtime_permissions project setting which allows users the ability to enable / disable the automatic request of runtime permissions that's done at start. This is done so that the app / game logic can itself request the permissions in a context-specific manner.

The project setting is enabled (set to true) by default to keep consistency with the current behavior.

Note that this PR builds on top of #202 to avoid duplicating the same logic across all the vendors modules.

[dsnopek]

Thanks! I did some testing with this, and it seems to be working great

I was using the 'meta-scene-sample', and if I uninstall it, then re-export with the "Automatically Request Runtime Permissions" unchecked, then the permission panel doesn't come up. However, if check it, and re-export, then the permission panel appears correctly.

I also experimented with manually requesting permissions. Both OS.request_permissions() and OS.request_permission("com.oculus.permission.USE_SCENE") will bring up the permission panel. However, I don't understand how I'm supposed to wait for the user to finish using the panel before moving on? Those functions return immediately with false, regardless of what the user chooses. Although, that's more of a Godot thing, rather than something this PR needs to solve :-)

[m4gr3d]

However, I don't understand how I'm supposed to wait for the user to finish using the panel before moving on? Those functions return immediately with false, regardless of what the user chooses.

That's the expected flow since the runtime permission dialog is async and we don't want to block waiting for it.
When the user accept (or deny) the permission request, a signal should be fired with the result.

See https://github.com/godotengine/godot/blob/27552a2f26adcdba579cd804197de3942e8cb0ec/platform/android/java_godot_lib_jni.cpp#L549 for reference.

[dsnopek]

See https://github.com/godotengine/godot/blob/27552a2f26adcdba579cd804197de3942e8cb0ec/platform/android/java_godot_lib_jni.cpp#L549 for reference.

Ah, the signal is on MainLoop! Because OS.request_permission() is on OS, I was looking for a signal there. I should have looked at the code :-)

This should probably be mentioned in the docs for OS.request_permission() too