Fix prefixed support for MSC4133 (#18875)

This fixes two bugs that affect the availability of MSC4133 until the
next spec release.

1. The servlet didn't recognise the unstable endpoint even when the
homeserver advertised it
 2. The HS didn't advertise support for the stable prefixed version

Would only have been a problem until the next spec release but it's nice
to have it work before then.

changelog.d/18875.bugfix

@@ -0,0 +1 @@
+Fix prefixed support for MSC4133.

synapse/rest/client/profile.py

@@ -109,6 +109,12 @@ class ProfileFieldRestServlet(RestServlet):
         self.hs = hs
         self.profile_handler = hs.get_profile_handler()
         self.auth = hs.get_auth()
+        if hs.config.experimental.msc4133_enabled:
+            self.PATTERNS.append(
+                re.compile(
+                    r"^/_matrix/client/unstable/uk\.tcpip\.msc4133/profile/(?P<user_id>[^/]*)/(?P<field_name>[^/]*)"
+                )
+            )
 
     async def on_GET(
         self, request: SynapseRequest, user_id: str, field_name: str

synapse/rest/client/versions.py

@@ -175,6 +175,7 @@ class VersionsRestServlet(RestServlet):
                     "org.matrix.simplified_msc3575": msc3575_enabled,
                     # Arbitrary key-value profile fields.
                     "uk.tcpip.msc4133": self.config.experimental.msc4133_enabled,
+                    "uk.tcpip.msc4133.stable": True,
                     # MSC4155: Invite filtering
                     "org.matrix.msc4155": self.config.experimental.msc4155_enabled,
                     # MSC4306: Support for thread subscriptions