[8.x] Use Generic Models on Factories

[aidan-casey]

This pull request adds generic models to Laravel factories. The benefit of this is that there will be better autocompletion and static analysis support for methods on the model (e.g., relationship methods, helper methods, etc.).

For example, the following code will autocomplete in IDEs that support it:

/**
 * @extends Factory<User>
 */
class UserFactory extends Factory
{
    protected $model = User::class;

    public function definition()
    {
        // ...
    }
}

UserFactory::new()
    ->create()
    ->getAuthIdentifier();

[fragkp]

Already closed: #38654

[aidan-casey]

Hey @fragkp, this PR takes a slightly different approach than the one you referenced and is more in line with the updates being made to collections, etc. Per a conversation with @nunomaduro on Twitter, I am hoping there is an opportunity for this to get merged. 🙂

[nunomaduro]

#39169