Add !reference support for workflow keyword

What does this MR do and why?

In the workflow:rules keyword, we need to allow the use of the !reference tag. So we add the accepted sub-type of the Ci JSON schema to accept an array of strings, which is what the !reference tag expect.

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Go to CI/CD editor
• Write any CI configuration file with the !reference tag inside the workflow:rules keyword. The simplest form would be:

.cond:
  rules:
    - if: '$FORCE_GITLAB_CI'

job1:
  script: echo hello
  rules:
    - !reference [.cond, rules]

• Hover on the !reference tag
• Notice there are no squiggly line! 🎉

MR acceptance checklist

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