Support weight widget GraphQL API

What does this MR do and why?

Resolves the backend portion of #362550 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Enable :work_items FF

   Feature.enable(:work_items)

2. Grab an issue and note its id and description.

If the issue's id is 460, its work item GID is gid://gitlab/WorkItem/449 (gid://gitlab/Issue/460 equally works.)

1. Run the sample GraphQL query

mutation {
  workItemUpdate(input: {id: "gid://gitlab/WorkItem/460", weightWidget: { weight: 2 } }) {
    workItem {
      id
      title
      widgets {
        ... on WorkItemWidgetWeight {
          weight
        }
      }
    }
    errors
  }
}

MR acceptance checklist

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