Evaluate/Add a remote interface for the initiator mode

[uweseimet]

The number of tools for the initiator mode is increasing, but there is no UI for this mode. It may make sense to add a protobuf remote interface for initiator mode operations. This interface essentially would have to expose functionality to run any SCSI command and to retrieve the results.
With such an interface there is the potential for implementing tools with a UI that can make use of initiator mode functionality, and these tools would not have to run on the Pi. Especially users not familiar with the command line could create hard drive/tape backups etc., with a UI running on a regular PC or Mac, for instance.

In the ideal case it will be possible to remotely switch between target and initiator mode. The emulation might be suspended while initiator mode functionality is in use.