Maintenance mode stacking support

The current implementation of maintenance mode for clusters supports only a single reason at a time, tracked using the simpleFields.REASON key. This restricts the functionality to a single actor and reason, which limits flexibility and coordination.

This proposal introduces a new design that allows multiple actors to independently place a cluster into maintenance mode for different reasons. We will extend the maintenance mode design to support multiple actors, each capable of independently adding or removing their own maintenance reason. The cluster will remain in maintenance mode as long as at least one active reason exists. Each reason will be associated with metadata such as the actor, reason, and timestamp. For backwards compatibility, the existing simpleFields.REASON will be retained and updated to reflect the most recent active reason. If a reason is removed, it will be replaced with the next most recent one. While legacy clients that remove the entire znode cannot be completely prevented, we will handle such cases gracefully and recommend migrating to an updated API that enables proper multi-actor maintenance handling.

Locate the maintenance mode implementation and trace how simpleFields.REASON is written, removed, and exposed through the maintenance API. Review the znode handling and existing legacy-client behavior first. Done means multiple actors can add and remove independent reasons, the cluster remains active while any reason exists, and simpleFields.REASON reflects the most recent active reason.

Written by the indexing model from the issue text.