client: add `RpcServiceT` middleware

[niklasad1]

jsonrpsee has a specific RPC middleware trait for the server and we should expose a similar API for the client as well for a uniform API. It may be useful for similar reasons but from the client-side.

Basically, the task is to move the RpcServiceT and associated layers except the Server's RpcService impl to core and change the return type to a generic one to work for both the client and server.