Drupal 8 custom REST resource for geting the server time

Custom module for Drupal 8 what create a simple custom REST resource for geting the server time using REST GET HTTP.

Reference

1. https://drupal.stackexchange.com/questions/276913/how-do-i-get-the-server-time-using-rest-get-http
2. https://www.drupal.org/docs/8/api/restful-web-services-api/custom-rest-resources
3. https://github.com/GiorgosK/time_service