Add support for model factories

[besanek]

This pull request introduce Callback Factory for Models. This is needed where we need to add attributes to the model via SCIM that are not sent by the client.

For example, if I want to add the id of the user who added the user, I can add this factory to the config.

//SCIMConfig
            'query' => null,
            'factory' => function () {
                $user = new User();
                $user->created_by = Auth::id();
                return $user;
            },
            'validations' => [

[arietimmerman]

Thanks!