add string key to materialize cursor

to support backends that can directly index instead of using an offset

authzed/api/materialize/v0/watchpermissionsets.proto

@@ -99,6 +99,8 @@ message Cursor {
   uint32 starting_index = 5;
   // completed_members is a boolean flag that indicates that the cursor has reached the end of the permission sets
   bool completed_members = 6;
+  // starting_key is a string cursor used by some backends to resume the stream from a specific point.
+  string starting_key = 7;
 }
 
 message LookupPermissionSetsRequest {
@@ -181,4 +183,4 @@ message LookupPermissionSetsRequired {
 message BreakingSchemaChange {
   // change_at is the revision at which a breaking schema event has happened.
   authzed.api.v1.ZedToken change_at = 1;
-}
+}