Add experimental and incomplete support for MSC4306: Thread Subscriptions. (#18674)

Implements:
[MSC4306](https://github.com/matrix-org/matrix-spec-proposals/blob/rei/msc_thread_subscriptions/proposals/4306-thread-subscriptions.md)
(partially)

What's missing:
- Changes to push rules

Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>

docker/configure_workers_and_start.py

@@ -327,6 +327,15 @@ WORKERS_CONFIG: Dict[str, Dict[str, Any]] = {
         "shared_extra_conf": {},
         "worker_extra_conf": "",
     },
+    "thread_subscriptions": {
+        "app": "synapse.app.generic_worker",
+        "listener_resources": ["client", "replication"],
+        "endpoint_patterns": [
+            "^/_matrix/client/unstable/io.element.msc4306/.*",
+        ],
+        "shared_extra_conf": {},
+        "worker_extra_conf": "",
+    },
 }
 
 # Templates for sections that may be inserted multiple times in config files
@@ -427,6 +436,7 @@ def add_worker_roles_to_shared_config(
         "to_device",
         "typing",
         "push_rules",
+        "thread_subscriptions",
     }
 
     # Worker-type specific sharding config. Now a single worker can fulfill multiple