Populate CI_MERGE_REQUEST_CHANGED_PAGE_* variables

What does this MR do?

Adds two predefined CI variables, CI_MERGE_REQUEST_CHANGED_PAGE_PATHS and CI_MERGE_REQUEST_CHANGED_PAGE_URLS. They are only populated on build with deployed environments for merge request Pipelines.

Why?

When a Review App is deployed, the configuration writer can automatically feed the pages known to be different post-merge in a Browser Performance or Accessibility test.

What does this MR not do?

• Implement any testing features. This solely populated the variables for use in arbitrary configuration.
• Populate changes for Review Apps outside of a merge request context. Because the merge request diff is necessary to determine what has changed, these variables are limited to that scope.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• ~~Label as security and @ mention @gitlab-com/gl-security/appsec~~
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team