Chain type field

[aglkm]

Added chain type field into get_status rpc as following:

{
  "id": 1,
  "result": {
    "Ok": {
      "chain": "main",
      "connections": 8,
      "protocol_version": 1000,
      "sync_status": "no_sync",
      "tip": {
        "height": 2813226,
        "last_block_pushed": "0001664af94b6e9585ac10c3925f3ab601165aca833f794ed676b5ee25f50c5f",
        "prev_block_to_last": "0001cf87025e4e515063263caee56ad3d7c6cdaf2e08751fdb1f10b5fe95f619",
        "total_difficulty": 2175334614969923
      },
      "user_agent": "MW/Grin 5.4.0-alpha.0"
    }
  }
}

Chain type: main
Protocol version: 1000
User agent: MW/Grin 5.4.0-alpha.0
Connections: 8
Chain height: 2813226
Last block hash: 0001664af94b6e9585ac10c3925f3ab601165aca833f794ed676b5ee25f50c5f
Previous block hash: 0001cf87025e4e515063263caee56ad3d7c6cdaf2e08751fdb1f10b5fe95f619
Total difficulty: 2175334614969923
Sync status: no_sync

Related to https://github.com/mimblewimble/grin/issues/3777