Add GraphQL mutation for subscribing to a work item

What does this MR do and why?

Add a mutation workItemSubscribe to allow users to subscribe to the work item notifications

Example mutation

mutation {
  workItemSubscribe(input: {id: "gid://gitlab/WorkItem/1", subscribed: true}) {
    workItem {
      widgets {
        ... on WorkItemWidgetNotifications {
          subscribed
        }
      }
    }
    errors
  }
}

Note: This mutation has been marked as alpha because work items are still under development.

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MR acceptance checklist

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Related to #412831 (closed)