list-untracked-repositories: Praefect sub-command to show untracked repositories

To help administrators in managing state of the praefect cluster a new praefect sub-command list-untracked-repositories was implemented.
On run it connects to all gitaly storages set in the configuration file and receives from each list of the repositories existing on the disk. Each repository then checked if it exists in the praefect database and if it is not the location of the repository is printed out in JSON format to the stdout of the process.

Closes: #3792 (closed)