Respect and retain existing shard zones when resharding with the same key.

This should include tests that make sure the zones are retained, that the zones are replaced if new zones are specified (including zones:[]), and if SERVER-76615 has been completed, combinations of this feature with explicit shard distribution. If SERVER-76615 has not been completed please add a note to that to do the testing of same-key resharding with existing zone and explicit distribution together.