adds a new event to denote a breaking schema change

A breaking schema changes require recomputing the snapshot
to rebuild the Materialize index, and in turn once
this is ready, the consumer will have to rebuild their
own indexes. This event is added to differentiate between
"schema changed" and "index ready after schema changed" events.

authzed/api/materialize/v0/watchpermissionsets.proto

@@ -81,6 +81,12 @@ message WatchPermissionSetsResponse {
     // the permission set snapshot needs to be rebuilt from scratch. This typically happens when the origin SpiceDB
     // cluster has seen its schema changed.
     LookupPermissionSetsRequired lookup_permission_sets_required = 3;
+
+    // breaking_schema_change is a signal that a breaking schema change has been written to the origin SpiceDB cluster,
+    // and that the consumer should expect delays in the ingestion of new changes,
+    // because the permission set snapshot needs to be rebuilt from scratch. Once the snapshot is ready, the consumer
+    // will receive a LookupPermissionSetsRequired event.
+    BreakingSchemaChange breaking_schema_change = 4;
   }
 }
 
@@ -159,8 +165,16 @@ message MemberReference {
 
 // LookupPermissionSetsRequired is a signal that the consumer should perform a LookupPermissionSets call because
 // the permission set snapshot needs to be rebuilt from scratch. This typically happens when the origin SpiceDB
-// cluster has seen its schema changed.
+// cluster has seen its schema changed, see BreakingSchemaChange event.
 message LookupPermissionSetsRequired {
   // required_lookup_at is the snapshot revision at which the permission set needs to be rebuilt to.
   authzed.api.v1.ZedToken required_lookup_at = 1;
+}
+
+// BreakingSchemaChange is used to signal a breaking schema change has happened, and that the consumer should
+// expect delays in the ingestion of new changes, because the permission set snapshot needs to be rebuilt from scratch.
+// Once the snapshot is ready, the consumer will receive a LookupPermissionSetsRequired event.
+message BreakingSchemaChange {
+  // change_at is the revision at which a breaking schema event has happened.
+  authzed.api.v1.ZedToken change_at = 1;
 }