Epic multiple boards switcher [RUN AS-IF-FOSS]

What does this MR do?

• Loads epic boards in board switcher dropdown
• Allow switching epic boards by selecting a board in dropdown options

Requires :epic_boards feature flag.

Progress of Epic boards feature development

Previous iterations	MR link
Index page	!53100 (merged)
Empty lists	!54169 (merged)

Screenshots (strongly suggested)

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #233434 (closed)