Create a RestorePartition RPC

We need an RPC that can restore an entire partition from backups created by Create a BackupPartition RPC (#6269 - closed)

We will want to eventually support restoring a specific repository in the partition. I think we can use our existing RestoreRepository RPC for this purpose. The reason for using this RPC over the per-repository one is that it will probably be much more efficient to restore the entire partition at once.

This RPC may have limited utility until we have some kind of backup index to find a backup to use based on some criteria. See Add a partition backup index (#6310 - closed)