[Network] [Brooklyn] VPN Connection does not return required parameters for its subresources

VPN Connection connects a VirtualNetworkGateway object to either another VirtualNetworkGateway object or a LocalNetworkGateway object. When I do a GET for a VPN connection object, these strongly typed objects are returned with only their ID fields populated. Thus when I try to issue a PUT to update the properties of the VPN connection, the operation fails because required properties of the strongly typed *NetworkGateway objects are missing.

The GET does not support $expand, so I have to get the connection object, then get the referenced gateway objects with separate REST calls to overwrite the references before I can do a PUT.

cc/ @DeepakRajendranMsft, @amarzavery, @johanste