Keyring: isolate the keyfetching mechanism

So I can call it from federation senders

synapse/crypto/keyring.py

@@ -273,9 +273,7 @@ class Keyring:
                 minimum_valid_until_ts=verify_request.minimum_valid_until_ts,
                 key_ids=list(key_ids_to_find),
             )
-            found_keys_by_server = await self._fetch_keys_queue.add_to_queue(
-                key_request, key=verify_request.server_name
-            )
+            found_keys_by_server = await self.fetch_keys(key_request)
 
             # Since we batch up requests the returned set of keys may contain keys
             # from other servers, so we pull out only the ones we care about.
@@ -302,6 +300,15 @@ class Keyring:
                 Codes.UNAUTHORIZED,
             )
 
+    async def fetch_keys(
+        self, key_request: _FetchKeyRequest
+    ) -> Dict[str, Dict[str, FetchKeyResult]]:
+        """Returns: {server name: {key id: fetch key result}}"""
+        found_keys_by_server = await self._fetch_keys_queue.add_to_queue(
+            key_request, key=key_request.server_name
+        )
+        return found_keys_by_server
+
     async def _process_json(
         self, verify_key: VerifyKey, verify_request: VerifyJsonRequest
     ) -> None: