Support branch ref in external repo CI/CD configuration file

What does this MR do?

Gitlab supports setting the CI/CD configuration file to an external repo with: .gitlab-ci.yml@mygroup/another-project but does not support setting a branch ref as part of it, although it is supported in the include syntax. This MR adds support for it with .gitlab-ci.yml@mygroup/another-project:ref. I'm open for a different syntax.

Related issues

I assume this is is requested here as well: #14376 (comment 551820835).

Check-list

Pre-merge

Consider the effect of the changes in this merge request on the following:

• Different pipeline types
• Non-canonical projects:
  • gitlab-foss
  • security
  • dev
  • personal forks
• Pipeline performance

I don't see a problem with any of those. The feature es there and this MR just exposes it in the web interface.

Post-merge

• Consider communicating these changes to the broader team following the communication guideline for pipeline changes