Expose user permissions on the WorkItem GraphQL type

What does this MR do and why?

Use expose_permissions in WorkItem Type. Necessary for #346132 (closed)

Related to #346132 (closed)

How to set up and validate locally

1. Run bundle exec rails console
2. Run Feature.enable(:work_items)
3. Execute a GraphQL query like

   {
     workItem(id: "gid://gitlab/WorkItem/260") {
       id
       userPermissions { deleteWorkItem readWorkItem updateWorkItem }
     }
   }

4. You should get a response like:

   {
     "data": {
       "workItem": {
         "id": "gid://gitlab/WorkItem/260",
         "userPermissions": {
           "deleteWorkItem": true,
           "readWorkItem": true,
           "updateWorkItem": true
         }
       }
     }
   }
   

MR acceptance checklist

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