WebSockets: improve handling of different message types #20
Description
This looks kinda insufficient: https://github.com/celestiaorg/optimint/blob/bfa0fa48319cefbc4c63c25e4655e862e5d3daa5/rpc/json/ws.go#L70-L73
Different message types should be handled properly. Possible message types are defined in RFC 6455, section 11.8.