Fix authorization for linked work items query

Related to #420605 (closed)

What does this MR do and why?

We are returning authorized items when resolving liked work items but this executes N+1 queries and breaks offset pagination (due to the authorized collection being an array).

To fix this, these changes move the authorization from the resolver LinkedItemsResolver to Types::WorkItems::LinkedItemType.

No adding a changelog because the linked work items feature is still behind the feature flag linked_work_items.

Screenshots or screen recordings

Click to expand

User with access to all linked items	User with access to public items only

How to set up and validate locally

1. Enable feature flags linked_work_items, okrs_mvc and work_items_mvc_2.

2. Visit a public project and create 4 objectives, mark the first one as related to the other 3 and turn confidentiality on for one of the related objectives:

   Click to expand

   

3. Visit http://127.0.0.1:3000/-/graphql-explorer and verify that the following query works with pagination and returns all related items

   Click to expand

   query getLinkedItems {
     workItem(id: "gid://gitlab/WorkItem/<objective_1_id>") {
       widgets {
         ... on WorkItemWidgetLinkedItems {
      	   linkedItems {
             pageInfo {
               startCursor
               endCursor
               hasNextPage
               hasPreviousPage
             }
             edges {
               node {
                 linkId
                 linkType
                 workItem {
                   title
                 }
               }
             }
           }
         }
       }
     }
   }

4. Login with a different user that is not a member of the group and try the same query. Verify that the confidential item is not included in the result

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.