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/utils.py

@@ -168,6 +168,7 @@ def default_config(name, parse=False):
         # background, which upsets the test runner.
         "update_user_directory": False,
         "caches": {"global_factor": 1},
+        "listeners": [{"port": 0, "type": "http"}],
     }
 
     if parse: