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

### 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_