WIP: Add Packagist project service

What does this MR do?

This MR adds a project service for auto-updating libraries on Packagist.

Are there points in the code the reviewer needs to double check?

• Currently, it shows up in the UI and works when I click the 'Test settings and save changes' button. However, that's the only way I'm able to get it to work: it doesn't run when I push a commit or a tag to the repository. What am I missing?
• I rarely write Ruby, so please heavily scrutinize these changes and let me know what I can improve.

Why was this MR needed?

Packagist is the main Composer repository and has become central to most of the PHP community's workflow. This MR simplifies publishing Composer libraries from GitLab instances.

Screenshots

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-com/support-forum/issues/2284