Add subepics feature to license and services

What does this MR do?

It adds subepics feature to licenses and updates Epic link services to check if subepics feature is available. It also adds the check to the controller and API.

This is the first iteration towards #37081 (closed) where we want to have epics for premium and epics with subepics structure for ultimate customers.

Once we make all necessary changes (the second MR should be focused on graphQL), the epics feature will be moved to premium in license.rb

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.