Make GDK workspace creation from monolith repo possible

Users need a simple option to create a workspace to start reviewing.

Proposal

For the MVP it is sufficient to implement creation of a workspace from the GitLab monolith repo, as described in gitlab-org/gitlab-development-kit#2158 (closed).

This is possible now that the monolith repo can be successfully cloned and will become even more performant once gitlab-org/gitlab#543982 (closed) is implemented.