Draft: Allow updating hierarchy widget with GraphQL

What does this MR do and why?

Related to #362213 (closed)

Adds Hierarchy input for workItemUpdateWidgets mutation

Queries

updateParent

mutation updateParent {
  workItemUpdateWidgets(input: { id: "gid://gitlab/Issue/2461", hierarchyWidget: { parentId: "gid://gitlab/Issue/1842"}}) {
    workItem {
      id
      description
      widgets {
        ... on WorkItemWidgetHierarchy {
              parent {
              id
              title
            }
            children {
              edges {
                node {
                  id
                  title
                }
             }
          }
        }
      }
    }
  }
}

updateChildren

mutation updateChildren {
  workItemUpdateWidgets(input: { id: "gid://gitlab/Issue/1842", hierarchyWidget: { childrenIds: ["gid://gitlab/Issue/2460"]}}) {
    workItem {
      id
      description
      widgets {
        ... on WorkItemWidgetHierarchy {
              parent {
              id
              title
            }
            children {
              edges {
                node {
                  id
                  title
                }
             }
          }
        }
      }
    }
  }
}

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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 #362213 (closed)