Add Epic to new filtered search in issue boards

What does this MR do and why?

Adds Epic as a filtering option with new filtered search on issue boards.

Behind :issue_boards_filtered_search feature flag.

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #337685 (closed)