Add Collection::Sorted class

What does this MR do?

• Adds the variable_inside_variable feature flag;
• Adds the Gitlab::Ci::Variables::Collection::Sorted class to perform a topological sort of an array of variables, and detect cyclic variable references, with 2 methods:
  1. errors to perform a validation of a runner variable array, gated by the new feature flag. It will return an error description if a cyclic reference is detected, nil otherwise;
  2. sort to perform topological sort of a runner variable array, gated by the new feature flag. It will return the original variables if a cyclic reference is detected;
• Adds a helper possible_var_reference? function to ExpandVariables to perform a quick check for $ to % so we can avoid more expensive operations if there is nothing to expand in a value;

Part of gitlab-runner#26345 (closed)

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