Issue #1850734 by klausi: Make serialization formats configurable.

.../rest/lib/Drupal/rest/EventSubscriber/RouteSubscriber.php

core/modules/rest/lib/Drupal/rest/Plugin/ResourceBase.php

...modules/rest/lib/Drupal/rest/Plugin/ResourceInterface.php

...st/lib/Drupal/rest/Plugin/rest/resource/DBLogResource.php

core/modules/rest/lib/Drupal/rest/RequestHandler.php

core/modules/rest/lib/Drupal/rest/Tests/CreateTest.php

core/modules/rest/lib/Drupal/rest/Tests/DeleteTest.php

core/modules/rest/lib/Drupal/rest/Tests/RESTTestBase.php

core/modules/rest/lib/Drupal/rest/Tests/ReadTest.php

core/modules/rest/lib/Drupal/rest/Tests/UpdateTest.php

core/modules/rest/rest.admin.inc