Create epics via epic board list

What does this MR do?

Allow user to create epic boards with a board list facet by providing a list.

Behind feature flag epic_boards.

To test:

• Enable feature flag epic_boards
• Create an epic board (see below)
• Create an epic board list in GraphiQL (see !55011 (merged) - make sure you get your list ID)
• Use the following GraphQL mutation:

mutation {
  boardEpicCreate(input: { groupPath: "group",
    boardId: "gid://gitlab/Boards::EpicBoard/7",
    title: "epic epic",
    listId: "gid://gitlab/Boards::EpicList/27"}) {
    errors
    epic {
      title
      labels {
        nodes {
          title
          color
        }
      }
    }
  }
}

Response

{
  "data": {
    "boardEpicCreate": {
      "errors": [],
      "epic": {
        "title": "epic epic",
        "labels": {
          "nodes": [
            {
              "title": "To Do",
              "color": "#F0AD4E"
            }
          ]
        }
      }
    }
  }
}

To create a new epic board:

• use the GraphQL query in !52258 (merged) OR
• view the index page for your group at https://localhost:3000/groups/group/-/epic_boards for one to be created automatically.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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 #233565 (closed)