WIP: Add graphQL mutation to set the epic of an issue

What does this MR do?

#36309 (closed)

Allows us to set the epic of an issue using GraphQL API endpoint issueSetEpic

mutation {
  issueSetEpic(input: {projectPath: "twitter/typeahead-js", iid: "9", epicId: 5}) {
    issue {
      iid
      epic {
        id
      }
    }
    epicIssue {
      epicIssueId
    }
  }
}

Screenshots

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

Closes #36309 (closed)