Add count field to WorkItemConnection

What does this MR do and why?

Add the count field to WorkItemConnection in GraphQL. This field wasn't included when introducing WorkItemType::Types because there was no use for it yet but now it's needed for work item children pagination (see Slack discussion (internal) for more context).

Example queries

query projectWorkItemsCount {
  workspace: project(fullPath: "group-a/project-a") {
    workItems {
      count
    }
  }
}

# {
#   "data": {
#     "workspace": {
#       "workItems": {
#         "count": 23
#       }
#     }
#   }
# }

query groupWorkItemsCount {
  workspace: group(fullPath: "group-b") {
    workItems {
      count
    }
  }
}

# {
#   "data": {
#     "workspace": {
#       "workItems": {
#         "count": 30
#       }
#     }
#   }
# }

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

✅

How to set up and validate locally

1. Enable feature flag with Feature.enable(:namespace_level_work_items)
2. Visit a group with a project and create work items at group and project levels by visiting http://gdk.test:3000/groups/<group_path>/-/work_items and http://gdk.test:3000/<project_full_path>/-/issues.
3. Use GraphQL explorer (https://gdk.test:3000/-/graphql-explorer) to test the example queries and verify that they return the correct counts.