Validate and cast string to integer in IntegerItem and/or IntegerData

Problem/Motivation

We use
\Drupal\Core\TypedData\Plugin\DataType\IntegerData
and
\Drupal\Core\Field\Plugin\Field\FieldType\IntegerItem
for integer id entity fields like user.uid.

However, when we load an entity from the database, we get the string version of these values, instead of the integer version. E.g. "123" instead of 123.

The 'integer' field type (IntegerItem) declares a property 'value' with data type 'integer' (IntegerData).
But:
- The property class is never instantiated, so any validation that the property could do does not happen.
- Even if the property class would be instantiated, it does nothing to validate the value.
- The field type class (IntegerItem) also does no validation.

Steps to reproduce

Create an entity (node or user).
Load the entity (make sure we don't get a cached value).
Call $entity->id().

Expected: Integer id like 123.
Actual: String id like "123".

Proposed resolution

The clean solution would be to instantiate the property class (IntegerData) in IntegerItem (or a base class), and let that property class do the casting.

If we care about performance, it would be simpler to let IntegerItem do the job by itself.

A proper cast + validation might look like this:

      $value = match (TRUE) {
        $value === NULL => NULL,
        is_int($value) => $value,
        is_string($value) && (string) (int) $value === $value => (int) $value,
        is_string($value),
        is_float($value) => throw new \InvalidArgumentException(sprintf(
          'Expected an integer value or NULL, found %s.',
          var_export($value, TRUE),
        )),
        default => throw new \InvalidArgumentException(sprintf(
          'Unexpected value of type %s found as entity id.',
          get_debug_type($value),
        )),
      };

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet