Display Epics in Epic Board [RUN-AS-IF-FOSS]

What does this MR do?

Displays epics in epic board lists.

Requires :epic_boards feature flag.

Notable changes to make boards more generic:

• Getter getIssuesByList renamed to getBoardItemsByList
• Getter getIssueById renamed getBoardItemById
• listIssues renamed to listItems
• State issues renamed to boardItems
• State issuesByListId renamed to boardItemsByListId

Example query to create a label list for local testing

mutation {
  epicBoardListCreate(input: {boardId: "gid://gitlab/Boards::EpicBoard/3", labelId: "gid://gitlab/GroupLabel/65"}) {
    list {
      id
    }
  }
}

Progress of Epic boards feature development

Previous iterations	MR link
Index page	!53100 (merged)
Empty lists	!54169 (merged)
Multiple epic boards switcher	!54399 (merged)
Create epic board	!54642 (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 #233438 (closed)