PlannedReparentShard should allow automatic replica selection even when cross cell boundaries

[aquarapid]

Currently PRS allows user to not specify a -new_master option. In this case, PRS will automatically choose a replica that is "reasonable" for promotion.

However, this logic only works for reparenting to a replica in the same cell as the current master. If you want to cross cells with the reparenting, you need to explicitly provide a -new_master tablet name.

We should remove this limitation, and allow PRS to take a destination cell as a parameter, and then use the existing internal logic to chose the most up-to-date replica in that cell.

[GuptaManan100]

Resolved by #16461