Define more headers as public, and split endpoints into separate library

[eddyashton]

This is a more-complete replacement for #2312.

This PR makes it so that public headers are included as ccf/ both locally and against the install, to distinguish them from the private headers (which are still installed, for now).

Additionally, it creates a ccf_endpoints library to break some of the include chains from app code into our implementation details, with the nice benefit of massively reducing build times for some kinds of change. I think its worth repeating this pattern for other parts of the code but I'm not sure what the next target is - I'd love to do it for the kv but not sure it'll work due to the heavy templating.

TODO:

• Write a CHANGELOG
• Trim the 3rdparty dependencies that don't need to be public (here)

[eddyashton]

This is part of #697. This doesn't yet include the split of 3rdparty into multiple folders - I have that on a separate branch but would prefer not to pollute this PR (it is almost purely a file rename, so easier to read in isolation).