Add start and due date widget to work items API

What does this MR do and why?

Allows querying start and due date for a work item using the GraphQL widget API

Step 2 for #365399 (closed)

How to set up and validate locally

1. Enable the work_items feature flag in the Rails console Feature.enable(:work_items).
2. Perform a query like:

{
  workItem(id: "gid://gitlab/WorkItem/549") {
    id
    widgets {
      type
      ... on WorkItemWidgetStartAndDueDate {
        startDate
        dueDate
      }
    }
  }
}

If the work item has a start or due date set in the DB, the value should be returned. I should be null otherwise.

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 #365399 (closed)