[8.x] Delegate lazy loading violation to method

[bonroyage]

This PR delegates the throwing of the LazyLoadingViolationException to a new method. This first checks the existence of a static callback for overriding the behaviour in the service provider, if not defined throw the exception as before. The user may override the method on their own models to customise the behaviour for specific models.

This would allow the user to enable the preventLazyLoading flag on all environments while being able to log a message instead of throwing an exception on production environments or in certain other scenarios:

// AppServiceProvider

public function boot()
{
    Model::handleLazyLoadingViolationUsing(function($model, $key) {
        \Log::warning("Lazy loaded relation [{$key}] on model [" . get_class($model) . "].");
    });
}

// A model

protected function handleLazyLoadingViolation($key)
{
    if(app()->isProduction()) {
        \Log::warning("Lazy loaded relation [{$key}] on model [" . get_class($this) . "].");
    } else {
        throw new LazyLoadingViolationException($this, $key);
    }
}

[bonroyage]

A possible further enhancement could be allowing this method to return false and preventing the relationship from being loaded without having thrown an exception. In that case the "attribute" would just return null just like the isRelation check does a little earlier and the behaviour is similar to the model events for saving, etc. where a false return value prevents the save() method from being called. I'd be happy to add it to this PR if wanted.

if ($this->preventsLazyLoading && $this->violatedLazyLoading($key) === false) {
    return;
}