Support activity-answer plugin configuration

Tested this and it works as expected. The class defining the ActivityAnswer should implements the buildConfigurationForm of the \Drupal\Core\Plugin\PluginFormInterface interface.

Thanks for testing!

Now that this issue is closed, please review the contribution record.

As a contributor, attribute any organization helped you, or if you volunteered your own time.

Maintainers, please credit people who helped resolve this issue.