Add an accuracy threshold to stop the trainings

[eyp]

Add another hyperparam, min_accuracy or accurracy_threshold to stop the training rounds when this value of accuracy is reached. This is not simple, because are the client nodes the ones that validates the training, so this accuracy value is not shared yet, probably the last accuracy value it should be returned by the clients along the model weights, then the server could do an average of the accuracy and use this value to check if the threshold has been reached.