Access host entity from RegistrationItemFieldItemlList

Problem/Motivation

I would expect to be able to do this:

$variation = ProductVariation::load($vid);
$variation->get('field_registration_types')->createHostEntity();

But I can't, there's no way of getting the host entity directly from the underlying entity.

I have to remember to go the long way around:

$handler = $this->entityTypeManager->getHandler('registration', 'host_entity');
$handler->createHostEntity($variation);

Proposed resolution

Add a createHostEntity() convenience method to RegistrationItemFieldItemlList.

User interface changes

None.

API changes

Method added to RegistrationItemFieldItemlList

Data model changes

None