Add support for 'raw' variables

What does this MR do?

Adds support for raw variables, that should be not expanded by Runner.

Why was this MR needed?

It's GitLab Runner's part of handling gitlab-ce#27436. Please look into issue for context.

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

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

closes #4696 (closed)