Start handlers for new media endpoints when media resource configured (#17483)

This is in response to issue #17473. 
Not all the necessary handlers to deal with media requests are started
now when configuring synapse to use a media worker as per the [example
config](https://element-hq.github.io/synapse/latest/workers.html#synapseappmedia_repository).
The new media endpoints introduced with authenticated media fall under
the `client` & `federation` handlers in synapse.
This PR starts up handlers for the new media endpoints if a worker has
been configured with only the `media` resource type.

### Pull Request Checklist

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before submitting your pull request -->

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- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
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* [X] [Code
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---------

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

2 files changed, 4 insertions, 2 deletions

diff --git a/synapse/federation/transport/server/__init__.py b/synapse/federation/transport/server/__init__.py
index 72599bb204..43102567db 100644
--- a/synapse/federation/transport/server/__init__.py
+++ b/synapse/federation/transport/server/__init__.py
@@ -271,6 +271,10 @@ SERVLET_GROUPS: Dict[str, Iterable[Type[BaseFederationServlet]]] = {
     "federation": FEDERATION_SERVLET_CLASSES,
     "room_list": (PublicRoomList,),
     "openid": (OpenIdUserInfo,),
+    "media": (
+        FederationMediaDownloadServlet,
+        FederationMediaThumbnailServlet,
+    ),
 }
 
 
diff --git a/synapse/federation/transport/server/federation.py b/synapse/federation/transport/server/federation.py
index b075a86f68..20f87c885e 100644
--- a/synapse/federation/transport/server/federation.py
+++ b/synapse/federation/transport/server/federation.py
@@ -912,6 +912,4 @@ FEDERATION_SERVLET_CLASSES: Tuple[Type[BaseFederationServlet], ...] = (
     FederationV1SendKnockServlet,
     FederationMakeKnockServlet,
     FederationAccountStatusServlet,
-    FederationMediaDownloadServlet,
-    FederationMediaThumbnailServlet,
 )