Include `io.element.thread` capability for MSC3440. (#11690)

changelog.d/11690.misc

+Update the `/capabilities` response to include whether support for [MSC3440](https://github.com/matrix-org/matrix-doc/pull/3440) is available.

synapse/rest/client/capabilities.py

@@ -73,6 +73,9 @@ class CapabilitiesRestServlet(RestServlet):
                 "enabled": self.config.registration.enable_3pid_changes
             }
 
+        if self.config.experimental.msc3440_enabled:
+            response["capabilities"]["io.element.thread"] = {"enabled": True}
+
         return 200, response