[WIP] Expand variables with values in CI config

What does this MR do?

Adds the ability to define more complex variables by means of variable composites.

this-job-name-123:
  except:
    variables:
    - $CI_COMMIT_MESSAGE =~ /Skip $CI_JOB_NAME/i
    - $STRING_VAR = Current job name is ${CI_JOB_NAME}
  script:
  - echo The job runs unless the commit message contains "Skip this-job-name-123"

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

• Replication of code between the Pattern and String Lexeme
• What is the desired behavior in case of a referenced variable not being defined (Current behavior is that the variable reference is removed from the original regexp or string)

Why was this MR needed?

• More flexibility should be added for the definitions of variables in CI jobs.
• This change was requested in #49534 (moved)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

@grzesiek