Use the standard entity handler pattern

Problem/Motivation

Registration uses this pattern:

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

This means there is only ever one host_entity handler, that defined for the registration entity type.

It should use this pattern:

$this->entityTypeManager->getHandler($entity->getEntityTypeId(), 'registration_host');
$host_entity = $handler->createHostEntity($entity);

This would allow sites to define different handlers (and thuse host entity wrappers) for different entity types. Which is the whole point of using a handler AFAIK.

Additionally, by namespacing the handler id as registration_host we prevent collision with a module that has its own idea of host entity.

Steps to reproduce

Handlers are documented here:
https://www.drupal.org/docs/drupal-apis/entity-api/handlers

The method signature is:

     * @param string $entity_type_id
     *   The entity type ID for this handler.
     * @param string $handler_type
     *   The handler type to create an instance for.
     *
     * @return object
     *   A handler instance.
     *
     * @throws \Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException
     */
    public function getHandler($entity_type_id, $handler_type);

Proposed resolution

1. Trigger a deprecation error when RegistrationHostEntityHandler is used on the registration entity type. Registration entities can't/shouldn't be hosts.

2. Use hook_entity_type_build() to set RegistrationHostEntityHandler as the default registration_host handler on all entity types.

3. Update all handler uses to use the new pattern.

4. Provide a BC shim using hook_entity_type_alter() and hook_module_implements_alter() in case the host_entity handler is overriden.

Remaining tasks

1. Proof of concept including a single change to use the new pattern in the registration entity class.
2. Fix all the other occurences.

User interface changes

None.

API changes

Using the provided handler for the registration entity type is deprecated.

Data model changes

None.