Include subepics in IssuesFinder if requested

What does this MR do?

It introduces a new param include_subepics into IssuesFinder. If this is set to true and epic_id param is present, the finder returns the issues of the epic and all its subepics.

This is the next iteration of searching issues by epics.

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.

Related #195180 (closed)