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.

src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/ServiceCollectionExtensions.cs

@@ -18,6 +18,9 @@ public static class ServiceCollectionExtensions
         services.AddOptions<OpcUaGlobalOptions>()
             .BindConfiguration("OpcUa");
 
+        services.AddOptions<MxGatewayGlobalOptions>()
+            .BindConfiguration("MxGateway");
+
         // WP-34: Register the factory for protocol extensibility
         services.AddSingleton<IDataConnectionFactory, DataConnectionFactory>();