Client Testsuite should utilise SerializationContextInitializers via configuration

The client testsuite makes extensive use of the ProtobufMetadataManagerImpl::registerMarshaller method to register ProtoStream marshallers, however this is no longer necessary as we now expose the SerializationContextInitializer via our server configuration.

Furthermore, it's also possible to directly use SerializationContextInitializer instances via the client configuration.

The client testsuite should be updated to utilise ^ mechanisms.