Support for Ambient Occlusion maps

I would like to propose the addition of support for ambient occlusion (AO) texture maps, so that OpenPBR may be used in real-time rasterizer contexts. AO maps are very common in these scenarios, but however are not common for path tracers. Therefore this should probably be allowed to be optional, with language suggesting how it can be used.

This would greatly reduce one of the primary friction points for adopting OpenPBR in a realtime context, especially very performance constrained scenarios where SSAO or path-traced results aren't possible.

No files, tests, or entry points are named in the issue. Start by clarifying the OpenPBR specification changes needed for optional ambient occlusion maps and how their real-time use should be described; done means an agreed specification and guidance for AO map support.

Written by the indexing model from the issue text.