lnrpc: add new min_relay_fee response to EstimateFee

We also start using a pattern of making new messages for types/units
instead of adding a suffix to a field.

lnrpc/walletrpc/walletkit_server.go

@@ -849,8 +849,11 @@ func (w *WalletKit) EstimateFee(ctx context.Context,
 		return nil, err
 	}
 
+	relayFeePerKw := w.cfg.FeeEstimator.RelayFeePerKW()
+
 	return &EstimateFeeResponse{
-		SatPerKw: int64(satPerKw),
+		SatPerKw:            int64(satPerKw),
+		MinRelayFeeSatPerKw: int64(relayFeePerKw),
 	}, nil
 }