Rake task to measure Import time and perform a cleanup afterwards

What does this MR do?

Perform a test project Import, measure the time, clean-up everything in the end.
Intended to be triggered on a regular basis and running on gitlabhq.

I introduced a down gem for simplicity (instead of using open-uri), as we use it in https://gitlab.com/gitlab-org/quality/performance as well.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

N/A

Security

N/A

Related to #196543 (closed)