Add keyboard models to XR_FB_render_model extension wrapper

[devloglogan]

#76 Adds in an XR_FB_render_model extension wrapper and corresponding OpenXRFBRenderModel node. There are two controller models and three keyboard models, but only the controller models are supported at this time (as tested on Quest 2 and 3).

To support at least the virtual keyboard model, the XR_META_virtual_keybaord extension will need a wrapper implemented.

Once keyboard models are supported, be sure to add their paths as suggestions to the get_suggested_render_model_path_names() function in openxr_fb_render_model.cpp.

[dsnopek]

I think the keyboard models we already see listed as valid paths might depend on the XR_FB_keyboard_tracking extension