Make node transfer staging path configurable

For vector indexes, there is a requirement for GSI that for codebook transfer to make the rpc server staging directory configurable.  Currently GSI only passes the StorageDir but the staging subdirectory is hard-coded in plasma. This is because plasma
takes the ownership of shard restoration. With vector, GSI will separately perform restore of codebook.

The change should not affect upgrades as staging is a local path on
destination and source node does not know about it. The staging path is of the following format : <@2i storageDir>/<staging subdirectory>.  The only case with upgrade I see is if rebalance fails and an offline upgrade happens, there is a possibility of stale data lying around. For this plasma, will ensure we remove any stale data.

cc: Shivansh Rustagi