gRFC L8: Separate internal-only and public portions of gRPC C++ wrapping

[Vijay Pai]

I have posted a gRFC as part of a process to more clearly demarcate which sections of the gRPC C++ wrapping are intended as public API and which are not, which formalizes a recent PR. Please post any discussion on this thread. This change does not affect code that uses the standard code generation process and in no way changes the documented public API. Thanks!

Regards,

Vijay Pai