Include rustls-ffi version parts in rustls.h header

[cpu]

Our API includes rustls_version() for getting a string at runtime with all of the version information relevant to the project (rustls-ffi version, rustls version, crypto backend versions, etc), but this is of little use when configuring downstream software that might want to determine if the librustls it has on hand is the right version for its needs.

At present downstream projects are left doing tricks like trying to detect the presence of symbols that only exist in certain versions. Instead, we should have #define's in our .h that can be checked mechanically.