Feature Request: PlannedReparentShard should allow cross-cell takeover without requiring tablet alias

[arthurschreiber]

Feature Description

PlannedReparentShard can only perform cross-cell takeovers when a tablet alias in another cell is explicitly passed in via the --new-primary flag.

I'm not sure what the historic reason for this behaviour is, but in our setup, we don't really care whether a planned failover picks a tablet in the same cell or in a different cell.

To support this use-case better, I propose we add a --target-cells option to PlannedReparentShard that allows in passing a list of cells (or cell aliases) from which the primary candidate will be picked. The option can default to the current primary's cell for backwards compatibility.

Use Case(s)

Picking a tablet ourselves and passing it in explicitly from the list of available tablets across all cells is not a good workaround, as it skips the sorting that ensures that the "most up to date" tablet is picked automatically.