feat(dcl): register MxGateway protocol in factory + config flatten + options

DataConnectionFactory registers 'MxGateway' -> MxGatewayDataConnection over the
real client; AddDataConnectionLayer binds MxGatewayGlobalOptions; DeploymentManager
FlattenConnectionConfig gains an MxGateway arm using the typed serializer. Factory
test confirms Create("MxGateway") returns the adapter.

tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/DataConnectionFactoryTests.cs

@@ -18,6 +18,16 @@ public class DataConnectionFactoryTests
         Assert.IsType<OpcUaDataConnection>(connection);
     }
 
+    [Fact]
+    public void Create_MxGateway_ReturnsMxGatewayAdapter()
+    {
+        var factory = new DataConnectionFactory(NullLoggerFactory.Instance);
+
+        var connection = factory.Create("MxGateway", new Dictionary<string, string>());
+
+        Assert.IsType<MxGatewayDataConnection>(connection);
+    }
+
     [Fact]
     public void Create_CaseInsensitive()
     {