Draft: Add default hierarchy restrictions

What does this MR do and why?

POC how usage of hierarchy restrictions might look like.

Related to #378058 (closed)

Example of query to get list of types which can be added to work item being displayed (related to #378890 (closed)):

query {
  workItem(id: "gid://gitlab/WorkItem/620") {
    widgets {
      ... on WorkItemWidgetHierarchy {
        allowedChildrenTypes {
          nodes{
            id
            name
          }
        }
      }
    }
  }
}

Example of mutation to add key result as a child to objective (related to #381817 (closed) and #381833 (closed)):

mutation {
  workItemCreate(input: {projectPath: "flightjs/Flight",title: "h3-test", workItemTypeId: "gid://gitlab/WorkItems::Type/92", hierarchyWidget: {parentId: "gid://gitlab/WorkItem/630"}}) {
    errors
  	workItem {
    	id
    	workItemType {
	      id
  	    name
    	}
    	widgets {
      	... on WorkItemWidgetHierarchy {
          parent {
            id
            title
          }
          availableChildrenTypes {
            nodes{
            	id
            	name
            }
          }
      	  }
    	}
    }
  }
}

Remove parent for a work item type (no change - works as is now - just by setting null parent_id):

mutation {
  workItemUpdate(input: {id: "gid://gitlab/WorkItem/634", hierarchyWidget: {parentId: null}}) {
    errors
  }
}

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