Add issues to graphQL group endpoint

What does this MR do?

This MR adds issues to group endpoint in graphQL.

Query example

{
  group(fullPath: "flightjs") {
    name
    issues(state: opened, milestoneTitle: "group-10.0", assigneeId: "none") {
      edges {
        node {
          iid
          title
        }
      }
    }
  }
}

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.

Backend for #197227 (closed)