adds optional revision in LookupPermissionSets

After the introduction of BreakingSchemaChange,
clients will be notified that an incompatible change has happened and
that they need rebuild their index. In order to do that, they need
to call LookupPermissionSets with the revision at which the breaking
event happened.

This introduces an optional revision in LookupPermissionSets to support
this use-case.

authzed/api/materialize/v0/watchpermissionsets.proto

@@ -106,6 +106,10 @@ message LookupPermissionSetsRequest {
   // the server will close the stream. If more permission sets are available, the consume should open a new stream
   // providing optional_starting_after_cursor, using the cursor from the last response.
   uint32 limit = 1;
+  // optional_at_revision specifies the client is requesting to lookup PermissionSets at a specific revision. It's
+  // optional, and if not provided, PermissionSets will be looked up at the current revision. The cursor always
+  // takes precedence in defining the revision when present.
+  authzed.api.v1.ZedToken optional_at_revision = 2;
   // optional_starting_after_cursor is used to specify the offset to start streaming permission sets from.
   Cursor optional_starting_after_cursor = 4;
 }