add parentId field to schema

This is somewhat debateable. Having a parentId field is wrong in itself. It should be a linked resource. But as the field is also present in the resource, it also needs to be part of the schema for consistency sake.

lib/api/v3/work_packages/schema/work_package_schema_representer.rb

@@ -114,6 +114,11 @@ module API
           schema :project,
                  type: 'Project'
 
+          schema :parent_id,
+                 type: 'Integer',
+                 required: false,
+                 writable: true
+
           schema_with_allowed_link :assignee,
                                    type: 'User',
                                    required: false,

spec/lib/api/v3/work_packages/schema/work_package_schema_representer_spec.rb

@@ -370,6 +370,16 @@ describe ::API::V3::WorkPackages::Schema::WorkPackageSchemaRepresenter do
       end
     end
 
+    describe 'parentId' do
+      it_behaves_like 'has basic schema properties' do
+        let(:path) { 'parentId' }
+        let(:type) { 'Integer' }
+        let(:name) { I18n.t('activerecord.attributes.work_package.parent') }
+        let(:required) { false }
+        let(:writable) { true }
+      end
+    end
+
     describe 'type' do
       it_behaves_like 'has basic schema properties' do
         let(:path) { 'type' }