Create a ListenerConfig object (#7681)

This ended up being a bit more invasive than I'd hoped for (not helped by
generic_worker duplicating some of the code from homeserver), but hopefully
it's an improvement.

The idea is that, rather than storing unstructured `dict`s in the config for
the listener configurations, we instead parse it into a structured
`ListenerConfig` object.

tests/unittest.py

@@ -229,7 +229,7 @@ class HomeserverTestCase(TestCase):
         self.site = SynapseSite(
             logger_name="synapse.access.http.fake",
             site_tag="test",
-            config={},
+            config=self.hs.config.server.listeners[0],
             resource=self.resource,
             server_version_string="1",
         )