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",
...