Restructure JSON-RPC directory and rename server config

[fedekunze]

given that we are supporting additional namespaces other than ethereum's now (eg: cosmos_ namespace #386), we should restructure the rpc directory in a more clear way:

rpc/
    ethereum/
       backend/
       namespaces/
          eth/ 
          ...
    cosmos/
       namespaces/
          cosmos/
    apis.go

Additionally, we should rename the EVMRPCConfig to JSONRPCConfig to reflect this change

[crypto-facs]

I will take this issue as I am currently restructuring this area for the debug namespace