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/DataConnectionFactory.cs

@@ -38,6 +38,13 @@ public class DataConnectionFactory : IDataConnectionFactory
         RegisterAdapter("OpcUa", details => new OpcUaDataConnection(
             new RealOpcUaClientFactory(globalOptions, _loggerFactory),
             _loggerFactory.CreateLogger<OpcUaDataConnection>()));
+
+        // MxGateway: gRPC to the MxAccess Gateway. The RealMxGatewayClient wraps the
+        // ZB.MOM.WW.MxGateway.Client package; per-connection config arrives via the
+        // flat details dict (see MxGatewayEndpointConfigSerializer).
+        RegisterAdapter("MxGateway", details => new MxGatewayDataConnection(
+            new RealMxGatewayClientFactory(_loggerFactory),
+            _loggerFactory.CreateLogger<MxGatewayDataConnection>()));
     }
 
     /// <summary>