[12.x] Use pendingAttributes of relationships when creating relationship models via model factories

[gdebrauwer]

When a relationship uses the withAttributes() method, then those attributes are currently not used when creating a relationship via a parent model factory

class Page extends Model
{
    public function currentVersion(): HasOne
    {
        return $this->hasOne(PageVersion::class)->withAttributes(['is_current' => true]);
    }
}

// The created page version will not have 'is_current' set to 'true'
$page = Page::factory()->hasCurrentVersion()->create();

// You currently have to explicitly set 'is_current' to true
$page = Page::factory()->hasCurrentVersion(['is_current' => true])->create();

This PR fixes that: the relationship's pending attributes will now be used by the model factories:

// The created page version will now have 'is_current' set to 'true'
$page = Page::factory()->hasCurrentVersion()->create();

// You still can override the values of the relationship's default attributes
// I added this behavior for backward compatibility in case someone those that
$page = Page::factory()->hasCurrentVersion(['is_current' => false])->create();