Expose the experimental appservice login flow to clients. (#8504)

changelog.d/8504.bugfix

+Expose the `uk.half-shot.msc2778.login.application_service` to clients from the login API. This feature was added in v1.21.0, but was not exposed as a potential login flow.

synapse/rest/client/v1/login.py

@@ -110,6 +110,8 @@ class LoginRestServlet(RestServlet):
             ({"type": t} for t in self.auth_handler.get_supported_login_types())
         )
 
+        flows.append({"type": LoginRestServlet.APPSERVICE_TYPE})
+
         return 200, {"flows": flows}
 
     def on_OPTIONS(self, request: SynapseRequest):