Expose the parts of jsonrpc_core_client necessary to create a custom transport

[phiresky]

I've built a transport that allows jsonrpc-client to work in the browser with wasm, but I think it's impossible to create such a transport outside of jsonrpc_core_client crate because some of the types used to build the transports are not exposed.

This includes:

• RpcMessage and all the variants
• RequestBuilder.* (call_request etc)
• probably others

It would be great if those types could be exposed to make it possible to create a custom transport

[shaggygi]

Not sure, but this might be something I'm also interested in. Early stages, but looking for a way to create custom transport like serial port. Understanding there are limitations like one way communications at a time, but okay with that. Thx.

[niklasad1]

hey, I don't expect anyone to work on that in this repo however we already got similar functionality that you want in https://github.com/paritytech/jsonrpsee

basically you have to implement two traits Sender and Receiver for the transport.

however, feel free to open a PR for that in this repo anyway :)