Merge #157224

157224: mmaprototype: move rebalanceStores to its own file r=tbg a=tbg

This PR refactors `rebalanceStores` by extracting the store-shedding logic from into a standalone `rebalanceStore` method on a new `rebalanceState` struct. The struct encapsulates rebalancing state (cluster state references, counters, configuration, and scratch variables), reducing the method's cyclomatic complexity and improving organization. More follow-up refactors are planned to straighten this up further and decompose it more, but this seemed like a good checkpoint.

There should be zero behavior changes in this PR.

Informs #157757.

Epic: CRDB-55052

Co-authored-by: Tobias Grieger <[email protected]>

Author: craig[bot]