[13.x] Add freshForUpdate() method to Eloquent models

[imKenjo18]

Following the addition of refreshForUpdate() in #61247, this PR introduces freshForUpdate() to Eloquent models as a fresh() counterpart.

Because refreshForUpdate() automatically re-loads all currently loaded relationships on the model, obtaining a fresh, locked instance without carrying over or re-querying those relations currently requires writing $model->unsetRelations()->refreshForUpdate() or falling back to manual queries like Model::query()->lockForUpdate()->find(...).

freshForUpdate() provides a clean shorthand that starts with an empty relations state (or only the relations explicitly requested):

// Before:
$order->unsetRelations()->refreshForUpdate();
// or:
$order = Order::query()->lockForUpdate()->find($order->id);

// After:
$order = $order->freshForUpdate();
// Or eager-loading only what you need:
$order = $order->freshForUpdate('items');

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!