Deprecate gdk init

What does this Merge Request do and why?

This MR deprecates the gdk init command as well as extracting its logic out into a command class and adds tests. We want to discourage the use of gdk init in favour of simply running git clone https://gitlab.com/gitlab-org/gitlab-development-kit.git because that's essentially all gdk init ever did.

This MR continues the work to replace gdk command line with a simple Shell script by stripping logic out from the gitlab-development-kit gem and moving inside the lib/ directory.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.