Allow projects API to receive order_by=star_count

What does this MR do and why?

Allow projects API to receive order_by=star_count

• This was an issue for Sourcegraph in pulling GitLab projects. This feature blocks Sourcegraph from including gitlab.com projects in sourcegraph.com/search.
• This was recently done for Groups in this MR.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Check out this MR changes and start GDK.
2. Visit /api/v4/projects?order_by=star_count.
3. The most starred projects should show up on the top.