Issue #3043455 by amateescu, jibran: Add an API for installing a fieldable entity type

.../lib/Drupal/Core/Entity/EntityDefinitionUpdateManager.php

...al/Core/Entity/EntityDefinitionUpdateManagerInterface.php

core/lib/Drupal/Core/Entity/EntityManager.php

...lib/Drupal/Core/Entity/EntityTypeEventSubscriberTrait.php

core/lib/Drupal/Core/Entity/EntityTypeListener.php

core/lib/Drupal/Core/Entity/EntityTypeListenerInterface.php

core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php

.../Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php

core/lib/Drupal/Core/Extension/ModuleInstaller.php

core/modules/system/system.install

...odules/entity_test/src/EntityTestDefinitionSubscriber.php

...est_update/src/EventSubscriber/EntitySchemaSubscriber.php

...al/KernelTests/Core/Entity/EntityDefinitionUpdateTest.php