[multistage][feature] allow broker to also run WorkerQuery engine

intermediate stage of the multi-stage engine is data locale unaware. so technically we can also register broker nodes as workers on intermediate stage engine.

Start by tracing the multi-stage engine's intermediate stage and how broker nodes and the WorkerQuery engine are registered. Confirm whether the intermediate stage is data-locale unaware as described, then determine the design and tests needed to allow broker nodes to run WorkerQuery there. Done means broker nodes can be registered and operate as workers without breaking existing multi-stage behavior.

Written by the indexing model from the issue text.