Add epic component to swimlanes board sidebar

What does this MR do?

Related to #37100 (closed).

This adds the epic selector to the swimlanes board sidebar. It uses the epic-select component to fetch epics and handle the selection logic.

Screenshots

Selecting and deselecting 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.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done