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

The change is a backport of the functionality implemented to resolve #3792 (closed) issue. A new sub-command for the praefect binary. 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.

Part of: #3792 (closed)

Changelog: added