Obtain/set headset refresh rate

[BastiaanOlij]

Different headsets run at different refresh rates which is a problem if we don't know about it because we want to keep our physics update rate in sync with our headset as best we can.

Unfortunately OpenXR does not expose this in the core specification but Facebook did provide a set of extensions:
https://www.khronos.org/registry/OpenXR/specs/1.0/html/xrspec.html#XR_FB_display_refresh_rate

Here we can enumerate a list of framerates supported by the headset, get its current setting and change it (the Quest 2 has two options so it can be compatible with the Quest 1).

Unfortunately seeing this is an extension we can't guarantee this feature is available. If not available we'll need to return a good default value.

[BastiaanOlij]

This was implementing through #88