Draft: parent child widget

What does this MR do and why?

An experiment which:

• adds basic support for widgets to WorkItems GraphQL API (to the work item query, and create mutation)
• adds parent/child widget plus description/related links widgets - these are not production ready, but should serve the purpose of getting idea how widgets might be used
• allows to set parent work item using generic WorkItem create mutation - the reason is that parent will be set when creating a work item (so it will be part of creation process). Because having all widgets to use one big mutation might be problematic, some widgets should use a separate mutation in future.

mutation {
  workItemCreate(input: {
    descriptionWidget: {text: "asdf"},
    parentLinkWidget:{parentId: "gid://gitlab/WorkItem/1"},
    projectPath: "flightjs/flight", title: "asdf",
    workItemTypeId: "gid://gitlab/WorkItems::Type/1"}) {
    
    workItem {
      id
      description
      widgets {
        type
        ... on WorkItemWidgetParentLink {
          parent {
            id
          }
        }
      }
    }
    errors
  }
}

• exposes work items widgets (description, related links):

query {
  workItem(id: "gid://gitlab/WorkItem/1") {
    id
    workItemType {
      id
    }
    title
    widgets {
      type
      ... on WorkItemWidgetDescription {
        text
      }
      ... on WorkItemWidgetLinks {
        relatedWorkItems {
          nodes {
          	id
          	issueLinkType
          }
        }
      }
      ... on WorkItemWidgetParentLink {
        parent {
          id
        }
        children {
          nodes {
            id
          }
        }
      }
    }
  }
}

This code is just an experiment, which may serve for inspiration how final/production-ready implementation might roughly look like, but nothing here is written in stone and can be changed.

Related to #358195 (comment 920166195)

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