Add new project readme experiment [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This adds an experiment about auto checking the "initialize with README" checkbox when creating a blank project/repository. It's also the first (almost, kinda) usage of the new gitlab-experiment gem, so there was some minor bug fixing in the gem to get to the place where this MR is.

https://gitlab.com/gitlab-org/gitlab/-/issues/285121

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

I'd like to make sure that where we added the tracking around initial repository writes is correct in terms of commit_count, and that it's in the right place to make sure it has as minimal an impact as possible to the performance of that aspect of gitlab -- since it does happen on every git push (and similar) action.