Upcast named arguments/named parameters in views

Problem/Motivation

Upcasting of route named Views parameters does not work. For instance, I'm expecting that a %user Views argument, which is set to validate as a valid user entity, will be upcasted to a user entity object. Running the following code should pass:

$account = \Drupal::routeMatch()->getParameter('user');
assert($account instanceof \Drupal\user\UserInterface);

Instead, the route matcher is still returning the raw argument as user ID.

Proposed resolution

Allow named parameters upcating to entities. Use the argument validation plugin to determine if a named parameter can be upcasted to its entity object.

Remaining tasks

None.

User interface changes

None.

API changes

Named Views arguments, validating as entities, are now upcasted as entity objects.

Data model changes

None.

Release notes snippet

If a named Views argument is validated as an entity then it will upcast as entity object.