Support :depends_on attribute in Collection::Item [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This is the fourth MR in a Russian-doll style chain of 8 MRs culminating with !48627 (merged). It adds a #depends_on property to Collection::Item so that we can quickly iterate over dependencies without recalculating them each time. This will be leveraged in the 6th MR in order to expand the variable value.

List of MRs

• MR 1: Make Collection::Sorted class take and output a Variables::Collection;
• MR 2: Add #sorted_collection method and #errors property to Variables::Collection;
• MR 3: Add indexing/lookup of variables to Variables::Collection;
• MR 4: Add support for depends_on property to Variables::Collection::Item;
• MR 5: Add support for raw property to Variables::Collection::Item;
• MR 6: Add an #expand_variables_collection function to ExpandVariables to perform a full expansion of a Variables::Collection, gated by the new project-scoped :variable_inside_variable feature flag. It will return the original object if a cyclic reference is detected;
• MR 7: Add error reporting to Gitlab::Ci::Pipeline::Seed::Build to let the user know when a problem occurs with variable expansion;
• MR 8: Change Ci::BuildRunnerPresenter#variables to leverage ExpandVariables#expand_variables_collection so that variables sent in the job response are expanded;

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

Part of gitlab-runner#26345 (closed)