Add support for operators when using the render delegate/usd imaging

[sirpalee]

Describe the feature you'd like Add support for using operators when using the render delegate and USD imaging.

There are several problems to solve:

• reading in procedural graphs as shaders easily
• using operators globally via render settings
• attaching operators to procedurals
• use of custom operators

[autodesk-oss-arnold-bot[bot]]

Issue synced internally to ARNOLD-13270

[BrianHanke]

Bumping this. I'm working on a custom version of Lentil and would love to add Solaris support.