Implementing OpenXR extensions

Khronos often adds new functionality to the OpenXR specification as extensions.

The general steps to implement an OpenXR extension in Monado are as follows.

Edit scripts/generate_oxr_ext_support.py. Usually you only need to add an entry to the EXTENSIONS list at the top.
Run the script python scripts/generate_oxr_ext_support.py.
Format the regenerated file with clang-format -i src/xrt/state_trackers/oxr/oxr_extension_support.h.
Add entry points for each new function in src/xrt/state_trackers/oxr/oxr_api_negotiate.c.
Add Monado internal prototypes for the new functions in src/xrt/state_trackers/oxr/oxr_api_funcs.h. The Monado implementations of OpenXR functions are prefixed with oxr_ and use the XRAPI_ATTR and XRAPI_CALL macros to satisfy calling conventions for all platforms.
Implement the oxr_ Monado internal functions in an appropriate source file src/state_trackers/oxr/oxr_api_*.c. Trivial functions can be implemented right there along with the usual parameter checks. More complex functions that use more internal Monado state should call functions implemented in the relevant oxr_*.c file (without _api_). The definitions for those functions go into src/xrt/state_trackers/oxr/oxr_objects.h.
Monado internal implementations of "objects" (think XrSession or XrHandTracker) go into src/xrt/state_trackers/oxr/oxr_objects.h.
Enums, defines and types go into src/xrt/include/xrt/xrt_defines.h. OpenXR types are typically not used outside of the oxr_api_* files, instead equivalents with the prefix XRT_ are defined here.