Add a 'ctl' admin api

[subotic]

The current way of initializing the different triplestores requires slightly different init scripts for each flavor of triplestore.

I would like to propose to add a 'ctl' admin API which would be available as soon as knora-api is started. It would then be used to control the knora-api service (start, stop, restart, status, backup, init triplestore, etc.). This ctl API would have a different authentication mechanism since the triplestore could not be available at this point, e.g., private/public keys.

There would be a tool knora-ctl used to interact with the ctl API.

The ctl API:

• would work with gRPC:
  • potentially need to transfer a large amount of data (backup, restore, upgrade)
  • a good small test case for trying out gRPC and gather experience
• be available as soon as knora-api starts
• allow starting, stopping, toggling read-only mode for /v1, /v2, and /admin routes
• provide backup, restore, db-init, repository upgrade functionality