Add hasEpic to issue type

What does this MR do and why?

Issue: #371083 (closed)

This adds hasEpic to the Issue type and indicates if an issue is related to an epic. It can be used in cases users have access to the issue but not the epic.

Screenshots or screen recordings

How to set up and validate locally

1. Find an issue with an epic, and one without and get the global ID (e.g. Epic.first.issues.first.id => 440, is the global ID: gid://gitlab/Issue/440)
2. Go to http://gdk.test:3000/-/graphql-explorer
3. Use the following GraphQL Query

query Issue {
	issueWithoutEpic: issue(id:"ISSUE_WITHOUT_EPIC_GLOBAL_ID") {
    id
    hasEpic
    epic {
      id
    }
  }
  

  issueWithEpic: issue(id:"ISSUE_WITH_EPIC_GLOBAL_ID"){
    id
    hasEpic
    epic {
      id
    }
  }
}
 

MR acceptance checklist

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