Allow to query a WorkItem's lockVersion via GraphQL

What does this MR do and why?

Required for !78414 (merged). Split into separate MR to make that one smaller. Allows to fetch a single WorkItem by GlobalID and the lockVersion on the returned item.

How to set up and validate locally

1. Run bundle exec rails console
2. Run Feature.enable(:work_items)
3. Execute a GraphQL query like

{
  workItem(id: "gid://gitlab/WorkItem/1") {
    id
    title
    lockVersion
  }
}

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 #346042 (closed)