Expose adminWorkItemLink permission in work items

What does this MR do and why?

Expose the adminWorkItemLink permission field in GraphQL to check the admin_work_item_link ability in work items.

This permission will be checked when displaying the Add button in the linked items widget:

How to set up and validate locally

1. Visit http://GDK_URL/-/graphql-explorer and test the following query:

query getWorkItemsPermissions {
  project(fullPath: "<project_full_path>") {
    workItems(iid: "<work_item_iid>") {
      edges {
        node {
          id
          userPermissions {
            adminWorkItemLink
          }
        }
      }
    }
  }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #424751 (closed)