Adaptive server selection support for multi-stage engine

Is adaptive server selection supported for the multi-stage engine? https://docs.pinot.apache.org/operators/operating-pinot/tuning/query-routing-using-adaptive-server-selection

cc @Jackie-Jiang

Start with the linked adaptive server selection documentation and inspect the multi-stage engine's server-selection entry points. Determine whether adaptive selection is supported there; done means providing a confirmed implementation status and, if unsupported, a defined path for adding it.

Written by the indexing model from the issue text.