Faster joins: Support for calling /federation/v1/state (#12013)

This is an endpoint that we have server-side support for, but no client-side support. It's going to be useful for resyncing partial-stated rooms, so let's introduce it.

tests/unittest.py

@@ -51,7 +51,10 @@ from twisted.web.server import Request
 
 from synapse import events
 from synapse.api.constants import EventTypes, Membership
+from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersion
 from synapse.config.homeserver import HomeServerConfig
+from synapse.config.server import DEFAULT_ROOM_VERSION
+from synapse.crypto.event_signing import add_hashes_and_signatures
 from synapse.federation.transport.server import TransportLayerServer
 from synapse.http.server import JsonResource
 from synapse.http.site import SynapseRequest, SynapseSite
@@ -839,6 +842,24 @@ class FederatingHomeserverTestCase(HomeserverTestCase):
             client_ip=client_ip,
         )
 
+    def add_hashes_and_signatures(
+        self,
+        event_dict: JsonDict,
+        room_version: RoomVersion = KNOWN_ROOM_VERSIONS[DEFAULT_ROOM_VERSION],
+    ) -> JsonDict:
+        """Adds hashes and signatures to the given event dict
+
+        Returns:
+             The modified event dict, for convenience
+        """
+        add_hashes_and_signatures(
+            room_version,
+            event_dict,
+            signature_name=self.OTHER_SERVER_NAME,
+            signing_key=self.OTHER_SERVER_SIGNATURE_KEY,
+        )
+        return event_dict
+
 
 def _auth_header_for_request(
     origin: str,