Return `is_admin` attribute in the GET /user endpoint for admins

Signed-off-by: Rémy Coutable <remy@rymai.me>

changelogs/unreleased/34325-reinstate-is_admin-for-user-api.yml

+---
+title: Return `is_admin` attribute in the GET /user endpoint for admins
+merge_request: 12811
+author:

doc/api/users.md

@@ -364,7 +364,7 @@ GET /user
 Parameters:
-- `sudo` (required) - the ID of a user
+- `sudo` (optional) - the ID of a user to make the call in their place
 ```
 GET /user

lib/api/users.rb

@@ -421,7 +421,16 @@ module API
         success Entities::UserPublic
       end
       get do
-        present current_user, with: sudo? ? Entities::UserWithPrivateDetails : Entities::UserPublic
+        entity =
+          if sudo?
+            Entities::UserWithPrivateDetails
+          elsif current_user.admin?
+            Entities::UserWithAdmin
+          else
+            Entities::UserPublic
+          end
+
+        present current_user, with: entity
       end
       desc "Get the currently authenticated user's SSH keys" do

spec/fixtures/api/schemas/public_api/v4/user/admin.json

+{
+  "type": "object",
+  "required": [
+    "id",
+    "username",
+    "email",
+    "name",
+    "state",
+    "avatar_url",
+    "web_url",
+    "created_at",
+    "is_admin",
+    "bio",
+    "location",
+    "skype",
+    "linkedin",
+    "twitter",
+    "website_url",
+    "organization",
+    "last_sign_in_at",
+    "confirmed_at",
+    "color_scheme_id",
+    "projects_limit",
+    "current_sign_in_at",
+    "identities",
+    "can_create_group",
+    "can_create_project",
+    "two_factor_enabled",
+    "external"
+  ],
+  "properties": {
+    "$ref": "full.json"
+  }
+}

spec/requests/api/users_spec.rb

@@ -943,11 +943,11 @@ describe API::Users do
           expect(response).to have_http_status(403)
         end
-        it 'returns initial current user without private token when sudo not defined' do
+        it 'returns initial current user without private token but with is_admin when sudo not defined' do
           get api("/user?private_token=#{admin_personal_access_token}")
           expect(response).to have_http_status(200)
-          expect(response).to match_response_schema('public_api/v4/user/public')
+          expect(response).to match_response_schema('public_api/v4/user/admin')
           expect(json_response['id']).to eq(admin.id)
         end
       end
@@ -961,11 +961,11 @@ describe API::Users do
           expect(json_response['id']).to eq(user.id)
         end
-        it 'returns initial current user without private token when sudo not defined' do
+        it 'returns initial current user without private token but with is_admin when sudo not defined' do
           get api("/user?private_token=#{admin.private_token}")
           expect(response).to have_http_status(200)
-          expect(response).to match_response_schema('public_api/v4/user/public')
+          expect(response).to match_response_schema('public_api/v4/user/admin')
           expect(json_response['id']).to eq(admin.id)
         end
       end