API: Expose 'developers_can_push' for branches

What does this MR do?

Adds support for the developers_can_push flag for the branches API. It also supports creating protected branches with that flag.

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

The API call requires an optional boolean parameter developers_can_push. If it is not either true or false, it should not update the value. I'm not sure if this is the right way to do it.

Right now it always returns 200. Maybe the PUT method should return different status codes depending if it really created or updated a branch.

What are the relevant issue numbers?

Closes #12735 (closed)

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 style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together