Support custom attributes on groups and projects

What does this MR do?

Extends custom attributes introduced in #13038 from users to groups and projects.

This contribution is sponsored by Siemens (/cc @bufferoverflow)

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

The implementation is the same as with users, so any performance/design/DB concerns should have already been addressed.

Why was this MR needed?

Custom attributes are useful for reporting purposes and building other services on top of GitLab, having them on groups and projects as well gives more flexibility.

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?