add ability to filter by state event type to admin state endpoint

docs/admin_api/rooms.md

@@ -385,6 +385,13 @@ The API is:
 GET /_synapse/admin/v1/rooms/<room_id>/state
 ```
 
+**Parameters**
+
+The following query parameter is available:
+
+* `type` - The type of room state event to filter by, eg "m.room.create". If provided, only state events
+    of this type will be returned. 
+
 A response body like the following is returned:
 
 ```json

synapse/rest/admin/rooms.py

@@ -23,6 +23,7 @@
 from typing import TYPE_CHECKING, List, Optional, Tuple, cast
 
 import attr
+from immutabledict import immutabledict
 
 from synapse.api.constants import Direction, EventTypes, JoinRules, Membership
 from synapse.api.errors import AuthError, Codes, NotFoundError, SynapseError
@@ -463,7 +464,23 @@ async def on_GET(
         if not room:
             raise NotFoundError("Room not found")
 
-        event_ids = await self._storage_controllers.state.get_current_state_ids(room_id)
+        state_filter = None
+        type = parse_string(request, "type")
+
+        if type:
+            if not type.startswith("m.room."):
+                raise SynapseError(
+                    HTTPStatus.BAD_REQUEST,
+                    "Type must be a room state event.",
+                )
+            state_filter = StateFilter(
+                types=immutabledict({type: None}),
+                include_others=False,
+            )
+
+        event_ids = await self._storage_controllers.state.get_current_state_ids(
+            room_id, state_filter
+        )
         events = await self.store.get_events(event_ids.values())
         now = self.clock.time_msec()
         room_state = await self._event_serializer.serialize_events(events.values(), now)