message: improve importcoinstake rpc output
diff: @@ -468,6 +468,7 @@ static RPCHelpMan importcoinstake()
  },
  RPCResult{RPCResult::Type::OBJ, "", "", {
  {RPCResult::Type::STR, "txid", "transaction id if import is successful."},
  + {RPCResult::Type::NUM, "nTime", "timestamp when coinstake is due to mint."},
  }},
  RPCExamples{
  HelpExampleCli("importcoinstake", "03000000")
  @@ -512,7 +513,10 @@ static RPCHelpMan importcoinstake()
  // add to in memory structure
  pwallet->m_coinstakes[timestamp] = tx;
  }
  - return tx->GetHash().GetHex();
  + UniValue result(UniValue::VOBJ);
  + result.pushKV("txid", tx->GetHash().GetHex());
  + result.pushKV("nTime", int(tx->nTime));
  + return result;
  },
  };
  }
  