[rllib] annotate public vs developer vs private APIs

[ericl]

Describe the problem

Based on user feedback I got, it would be helpful to annotate methods to describe their API stability.

For example, @PublicAPI could be used for very stable APIs (e.g., model), which have basically never been broken. @DeveloperAPI could be used for best-effort stable internal APIs that are exposed for custom algorithms, and no annotation would mean the API is subject to change.
We could also have @Experimental for public APIs that aren't quite ready for commitment to stability yet.