Add group.workItem query to GraphQL API

What does this MR do and why?

New field allows to fetch a single work item associated with a group by specifying it's IID in the group. Feature behind the namespace_level_work_items feature flag.

How to set up and validate locally

1. Enable the feature flag in Rails console Feature.enable(:namespace_level_work_items)
2. Create a group level work item with the following query at http://127.0.0.1:3000/-/graphql-explorer so you can fetch a work item in the next query

   mutation {
      workItemCreate(input: {namespacePath: "flightjs", title: "test group level work item", workItemTypeId: "gid://gitlab/WorkItems::Type/1"}) {
        errors
        workItem {
          id
          iid
        }
      }
    }

3. Fetch a wok item by IID

   {
      group(fullPath: "flightjs") {
        name
        workItem(iid: "<iid_returned_in_previus_mutation>") {
          nodes {
            id
            iid
            title
          }
        }
      }
    }

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 #424662 (closed)