Allow including custom attributes in API responses

What does this MR do?

This adds a with_custom_attributes parameter to directly include custom attributes (introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13038 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14593) in the response on queries for users/projects/groups. The following endpoints are affected:

• GET /users
• GET /users/:id
• GET /users/:user_id/projects
• GET /projects
• GET /projects/:id
• GET /projects/:id/forks
• GET /groups
• GET /groups/:id
• GET /groups/:id/projects
• GET /groups/:id/subgroups

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

• The shared specs in spec/support/shared_examples/requests/api/status_shared_examples.rb only test the common /$COLLECTION and /$COLLECTION/:id endpoints, if necessary I can add more specs for the others
• The API docs don't include example responses, not sure if this is necessary either (the docs already have inconsistent formats, and as I remember there are ongoing discussions about switching to auto-generated docs anyway)

Why was this MR needed?

If you want to get the custom attributes for a large number of resources, an additional request is needed per resource. As an example:

Getting the information of 1000 users, I need to do

• 10 users HTTP requests, each for a page of users with page size 100
• 1000 custom attribute requests for each single user

=> sums up to 1010 HTTP requests instead of 10

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
• Internationalization required/considered

What are the relevant issue numbers?