Allow to find epics by the given time frame

What does this MR do?

Allow to find epics by the given time frame

On the epics roadmap page (see #3559 (closed)) we need to display epics in the given time frame. This patch adds an additional filter to the Epic finder which allows to filter only epics which overlap with the given time frame, epics which do not have start and end date set are intentionally ignored.

This filter is not used anywhere yet, but it will be used in the MR for #3559 (closed). This patch is a separate MR to avoid overgrowing and mixing too many things together in the frontend patch for #3559 (closed).

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

#3559 (closed)