Add chain type parameter into get_status rpc

[aglkm]

I didn't find a way to get the information from the RPC what is the chain type the node currently running: mainnet or testnent. It would be good to have this information in get_status RPC request.

E.g.:

{ "jsonrpc": "2.0", "method": "get_status", "params": [], "id": 1 } { "id": 1, "jsonrpc": "2.0", "result": { "Ok": { "chain": "main" "protocol_version": "2", "user_agent": "MW/Grin 2.x.x", "connections": "8", ...