Move transport-specific handlers to sub-packages in `jsonrpc`

[joshklop]

jsonrpc implements the (transport-agnostic) JSON-RPC v2 spec. Dependents should not need to rely on transport code in net/http and nhooyr.io/websocket without explicit requesting it.

[omerfirmak]

Is this still relevant?

[joshklop]

Yes. I was planning on doing this after #1389, but ended up focusing on other things in the meantime.

[joshklop]

Noticed while working on this: we are using the same namespace, subsystem, and name for all RPC method counters, across all servers. Do we need to aggregate them somehow?