Add support for variable for pages.expire_in

What does this MR do and why?

Currently pages.expire_in does not support variables and because of which we can not pass rule based different values for it. So this MR is to add support for variable for pages.expire_in.

Related: #492289 (closed)

Changelog: added

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

This is a simple example. The DURATION variable can have different values based on different rules.

1. Create a GitLab Pages website from a project template and in step 3 of template selection, select Pages/Plain HTML template.
2. Update .gitlab-ci.yml file with below configuration:

review-pages:
  image: busybox
  stage: deploy
  script:
    - echo "The site will be deployed to $CI_PAGES_URL"
  pages: 
    publish: public
    expire_in: $DURATION
  variables:
    DURATION: 7d
  rules:
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

3. Run New pipeline and it should succeed with having pages:deploy job.

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