The new cpp-lib template should be "hybrid ready" with exposed USE_FABRIC constants, etc

[jonthysell]

Summary

Make sure we conditionally add the right constants in the lib build files and add examples in the code to show when a dev might need to use them

Motivation

Right now we assume only our internal libs build conditionally paper vs fabric. We need to extend this to external libs so they can build appropriately too.

Basic Example

No response

Open Questions

No response