feat: nested services

[Frando]

(draft)

Adds Client::map and MappedClient to easily perform requests on embedded (nested) services.
This allows composing services within a single protocol / connection.

Also adds a nested argument to the rpc macro to allow doing this via the macro. If nested is set, no WithChannels conversions are created, instead the message contains the message specified via the nested argument.

See the nested example.

[rklaehn]

Not sure if this is the right way to go. It adds quite a bit of complexity. Ideally you would be able to mash together lots of different services on a single endpoint if the enum discriminator was not just a varint but something that actually describes the request semantically.

I would rather follow the latter approach, which is what I prototyped in https://crates.io/crates/irpc-schema .

[Frando]

Yeah, let's leave it as a draft for a bit, I also wanna play with irpc-schema more before committing to this approach.