Support filtering by epics in IssuesFinder

What does this MR do?

It adds a possibility to filter by epics to the issue finder. This backend change is needed in order to support filtering by epics in the issues list.

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.

Issue: #195178 (closed)