Add startAndDueDate widget to workItemUpdate mutation

What does this MR do and why?

Allows updating start and due date for a work item via the GraphQL API

How to set up and validate locally

1. Enable the work_items feature flag Feature.enable(:work_items).
2. Update start and due date for a work item using a query like:

mutation {
  workItemUpdate(
    input: { id: "gid://gitlab/WorkItem/549", startAndDueDateWidget: { startDate: "2030-01-01", dueDate: "2030-02-01" } }
  ) {
    errors
    workItem {
      id
      widgets {
        type
        ... on WorkItemWidgetStartAndDueDate {
          startDate
          dueDate
        }
      }
    }
  }
}

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)