Server API (and crate seperation)

[yusdacra]

It's probably best to seperate the stuff into seperate crates. Probably:

• harmony_api: holds generated api code (control server / client code generation via a feature)
• harmony_api_macros: macros we'll use in api crates
• harmony_client_api: client api methods, client: &Client should be made generic using something like AsMut<...Service> instead
• harmony_federation_api: same as above but for server
• harmony_client_stateless: stateless client feat: seperate client frameworks #3
• harmony_client_stateful: stateful client feat: seperate client frameworks #3

and a harmony crate which re-exports these controlled with feature gates. Pretty much like ruma structure.

[yusdacra]

Decided this isn't really needed nor useful since we use macros and generated API code, which makes our code more concise. We can still use feature gates to disable client etc.