[rpc api]: reduce dependency tree

[niklasad1]

Currently, the jsonrpc RPC API from jsonrpsee introduces some needless dependencies which should be fixed shortly.

In addition the crate itself brings in sc-chain-spec which in turn brings in sc-network and it has lots of dependencies (libp2p and so on) which are not needed just because a couple of types are needed in the system RPC API.

Thus, would be nice to extract these types or even duplicate them just to avoid having these extra dependencies.

The typical use-case is that one wants to re-use the RPC API from substrate in some client library/application to avoid duplicating the entire API or import half of the dependencies in substrate.