Expose more attributes to `GET /projects/:id`

As reported in https://gitlab.com/gitlab-org/gitlab-ce/issues/32226, Composer needs the default_branch attribute but this is not currently exposed in the unauthenticated GET /projects/:id API endpoint.

I propose to expose a bunch of attributes in the unauthenticated endpoint:

• description
• default_branch
• tag_list
• ssh_url_to_repo
• created_at
• last_activity_at
• star_count
• forks_count
• open_issues_count
• statistics

This should solve #32226 (closed). /cc @mlncn