Issue #3491874 by dieterholvoet: Add attribute for Required plugins

composer.json

required_api.info.yml

required_api.module

src/Annotation/Required.php

src/Attribute/Required.php

src/Form/RequiredDefaultPluginForm.php

src/Plugin/Required/RequiredBase.php

src/Plugin/Required/RequiredDefault.php

src/Plugin/RequiredPluginInterface.php

src/RequiredApiFormErrorHandler.php

src/RequiredManager.php

src/TypedData/RequiredApiTypedDataManager.php

tests/modules/required_api_test/required_api_test.info.yml

tests/modules/required_api_test/required_api_test.module

tests/src/Functional/RequiredApiTest.php

tests/src/Functional/RequiredApiTestBase.php

tests/src/Unit/RequiredManagerTest.php

tests/src/Unit/RequiredTestTrue.php