Add ancestors field to hierarchy widget

Related to #421615 (closed)

What does this MR do and why?

In order to implement Work Items: Ancestry Widget (&11197 - closed) we need to be able to access the ancestors of a work item, meaning the list of consecutive parents starting from the top-level.

This list is returned in the GraphQL field ancestors that is included in the existing widget WorkItemWidgetHierarchy. The list of work items is authenticated for the current user and if any inaccessible items are present the list is truncated at the last visible one, starting from the bottom.

The work items' namespace (or project) is preloaded to avoid N+1 queries during authorization. We also use lookahead to prevent N+1s when fetching the ancestors' own fields.

Screenshots or screen recordings

This functionality hasn't been implemented in the UI but it can be tested using GraphQL explorer:

Click to expand

How to set up and validate locally

1. Using rails console enable the feature flag

   Feature.enable(:namespace_level_work_items)

2. Create two groups, one public and one private

   user = User.first
   private = Group.create!(name: 'Private Group', path: 'private_group', visibility_level: Gitlab::VisibilityLevel::PRIVATE)
   public = Group.create!(name: 'Public Group', path: 'public_group', visibility_level: Gitlab::VisibilityLevel::PUBLIC)

3. Create 3 work items with type Epicz in the public group, and one in the private group

   wi_type = WorkItems::Type.find_by(base_type: 'epic')
   child = WorkItem.create!(title: "Child", namespace: public, author: user, work_item_type: wi_type)
   ancestor1 = WorkItem.create!(title: "Ancestor 1", namespace: public, author: user, work_item_type: wi_type)
   ancestor2 = WorkItem.create!(title: "Ancestor 2 (cross-group)", namespace: private, author: user, work_item_type: wi_type)
   ancestor3 = WorkItem.create!(title: "Ancestor 3", namespace: public, author: user, work_item_type: wi_type)

4. Create the hierarchy by adding each epic as a parent of the previous one

   WorkItems::ParentLink.create!(work_item: child, work_item_parent: ancestor1)
   WorkItems::ParentLink.create!(work_item: ancestor1, work_item_parent: ancestor2)
   WorkItems::ParentLink.create!(work_item: ancestor2, work_item_parent: ancestor3)

5. Logged in as the root user visit 127.0.0.1:3000/-/graphql-explorer and query the ancestors using the following query:

   Click to expand

   query getAncestors {
     workItem(id: "gid://gitlab/WorkItem/<child_id>") {
       title
       widgets {
         ...on WorkItemWidgetHierarchy {
           ancestors {
             edges {
               node {
                 title
               }
             }
           }
         }
       }
     }
   }

6. Verify that the response includes the 3 ancestors in the correct order:

   Click to expand

   {
     "data": {
       "workItem": {
         "title": "Child",
         "widgets": [
           {
             "ancestors": {
               "edges": [
                 {
                   "node": {
                     "title": "Ancestor 3"
                   }
                 },
                 {
                   "node": {
                     "title": "Ancestor 2 (cross-group)"
                   }
                 },
                 {
                   "node": {
                     "title": "Ancestor 1"
                   }
                 }
               ]
             }
           },
         ]
       }
     }
   }

7. Login with a different (non-admin) user and execute the same query. This time the response should only include Ancestor 1 because the list is truncated at the last visible ancestor:

   Click to expand

   {
     "data": {
       "workItem": {
         "title": "Child",
         "widgets": [
           {
             "ancestors": {
               "edges": [
                 {
                   "node": {
                     "title": "Ancestor 1"
                   }
                 }
               ]
             }
           }
         ]
       }
     }
   }

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