Create namespace.workItemStateCounts API

To have a more generic frontend and no longer distinguish between Group and Project, we want to add a new API on namespace to query workItemStateCounts. The API for a single work item already exists.

query workItemsStateCounts {
  namespace(fullPath: "gitlab-org") {
    workItemStateCounts
  }
}

The API should be the same as we currently have on groups and projects.