Add license attribute to the Projects API endpoint

What does this MR do?

This MR exposes the license of the given project in the Projects API. Requested by Andrew Nesbitt, I think this is a pretty obvious addition.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together