Allow selecting "prejoin" events by state keys (#14642)

* Declare new config

* Parse new config

* Read new config

* Don't use trial/our TestCase where it's not needed

Before:

```
$ time trial tests/events/test_utils.py > /dev/null

real	0m2.277s
user	0m2.186s
sys	0m0.083s
```

After:
```
$ time trial tests/events/test_utils.py > /dev/null

real	0m0.566s
user	0m0.508s
sys	0m0.056s
```

* Helper to upsert to event fields

without exceeding size limits.

* Use helper when adding invite/knock state

Now that we allow admins to include events in prejoin room state with
arbitrary state keys, be a good Matrix citizen and ensure they don't
accidentally create an oversized event.

* Changelog

* Move StateFilter tests

should have done this in #14668

* Add extra methods to StateFilter

* Use StateFilter

* Ensure test file enforces typed defs; alphabetise

* Workaround surprising get_current_state_ids

* Whoops, fix mypy

synapse/types/state.py

@@ -118,6 +118,15 @@ class StateFilter:
             )
         )
 
+    def to_types(self) -> Iterable[Tuple[str, Optional[str]]]:
+        """The inverse to `from_types`."""
+        for (event_type, state_keys) in self.types.items():
+            if state_keys is None:
+                yield event_type, None
+            else:
+                for state_key in state_keys:
+                    yield event_type, state_key
+
     @staticmethod
     def from_lazy_load_member_list(members: Iterable[str]) -> "StateFilter":
         """Creates a filter that returns all non-member events, plus the member
@@ -343,6 +352,15 @@ class StateFilter:
             for s in state_keys
         ]
 
+    def wildcard_types(self) -> List[str]:
+        """Returns a list of event types which require us to fetch all state keys.
+        This will be empty unless `has_wildcards` returns True.
+
+        Returns:
+            A list of event types.
+        """
+        return [t for t, state_keys in self.types.items() if state_keys is None]
+
     def get_member_split(self) -> Tuple["StateFilter", "StateFilter"]:
         """Return the filter split into two: one which assumes it's exclusively
         matching against member state, and one which assumes it's matching