Add 403 error response to federation/v1/state_ids (#1926)

fixes #1912 by adding a 403 error response to /_matrix/federation/v1/state_ids/{roomId}

Signed-off-by: Srinjoy Sen Chowdhury  <srinjoysen123@gmail.com>

changelogs/server_server/newsfragments/1926.clarification

@@ -0,0 +1 @@
+Add 403 error response to `/_matrix/federation/v1/state_ids/{roomId}`.

data/api/server-server/events.yaml

@@ -140,6 +140,19 @@ paths:
                 required:
                   - auth_chain_ids
                   - pdu_ids
+        "403":
+          description: |-
+            The requesting host is not in the room, or is excluded from the room via `m.room.server_acl`.
+          content:
+            application/json:
+              schema:
+                $ref: ../client-server/definitions/errors/error.yaml
+              examples:
+                response:
+                  value: {
+                    "errcode": "M_FORBIDDEN",
+                    "error": "You do not have permission to access the state at the requested event."
+                  }
         "404":
           description: |-
             The given `event_id` was not found or the server doesn't know about the state at