Fix IL2CPP OpenXR hook trampoline

[andybak]

Fixes an Android IL2CPP launch failure in Unity OpenXR Extensions.

On Quest, Open Brush was hanging during launch because OpenXR initialization threw NotSupportedException when IL2CPP
tried to marshal FeatureBase.Intercepted_xrGetInstanceProcAddr. Although the method had MonoPInvokeCallback, it
lived inside a generic class, so the closed generic feature types still failed at runtime.

This moves the xrGetInstanceProcAddr trampoline into a non-generic helper with a single MonoPInvokeCallback entry
point, while keeping per-feature handlers registered from FeatureBase. That lets multiple OpenXR extension features
hook function lookup without requiring IL2CPP to marshal a generic-class callback.