[9.x] Address Dynamic Relation Resolver inconsiency issue with extended Models

[vmcvlad]

This PR aims to address the inconsistency issues with child Eloquent Models not inheriting their parents' dynamically registered relations, an issue which I attempted to explain in detail in #44741.

I have also added an in-line comment with an alternative approach to implementing the relationResolver() method.

[vmcvlad]

Alternative implementation:

public function relationResolver($key)
{
    $class = static::class;

    do {
        $resolver = static::$relationResolvers[$class][$key] ?? null;
    } while (!$resolver && $class = get_parent_class($class));

     return $resolver;
}