Create a BackupPartition RPC

Create a RPC called BackupPartition that backs up an entire partition and saves it to the configured backup sink (filesystem or object storage).

Now that transactions support being created on the entire partition we can add a partition scope to RPCs so that the transaction middleware knows to open a full partition transaction.

Once the middleware has created the full partition transaction the new RPC only needs to tar the contents of the read snapshot via the snapshot root path

The tar should be saved based off the read snapshot LSN so that we know where in the log it was for replaying archived WAL entries.

It's possible that the list of relative paths in the partition needs persisting too.