Expose confidential field for work items

This is same as Issue's confidential field.

app/graphql/types/work_item_type.rb

@@ -6,6 +6,8 @@ class WorkItemType < BaseObject
 
     authorize :read_work_item
 
+    field :confidential, GraphQL::Types::Boolean, null: false,
+          description: 'Indicates the work item is confidential.'
     field :description, GraphQL::Types::String, null: true,
           description: 'Description of the work item.'
     field :id, Types::GlobalIDType[::WorkItem], null: false,

doc/api/graphql/reference/index.md

@@ -18530,6 +18530,7 @@ Represents vulnerability letter grades with associated projects.
 | Name | Type | Description |
 | ---- | ---- | ----------- |
+| <a id="workitemconfidential"></a>`confidential` | [`Boolean!`](#boolean) | Indicates the work item is confidential. |
 | <a id="workitemdescription"></a>`description` | [`String`](#string) | Description of the work item. |
 | <a id="workitemdescriptionhtml"></a>`descriptionHtml` | [`String`](#string) | The GitLab Flavored Markdown rendering of `description`. |
 | <a id="workitemid"></a>`id` | [`WorkItemID!`](#workitemid) | Global ID of the work item. |

spec/graphql/types/work_item_type_spec.rb

@@ -11,6 +11,7 @@
 
   it 'has specific fields' do
     fields = %i[
+      confidential
       description
       description_html
       id

spec/requests/api/graphql/work_item_spec.rb

@@ -41,6 +41,7 @@
         'lockVersion' => work_item.lock_version,
         'state' => "OPEN",
         'title' => work_item.title,
+        'confidential' => work_item.confidential,
         'workItemType' => hash_including('id' => work_item.work_item_type.to_gid.to_s),
         'userPermissions' => { 'readWorkItem' => true, 'updateWorkItem' => true, 'deleteWorkItem' => false }
       )