refactor(policy): add toString() to common package

package eu.teraflow.policy.common;

import java.util.List;

import java.util.stream.Collectors;

public class Util {

    private Util() {}

    public static <T> String toString(List<T> list) {

        return list.stream().map(T::toString).collect(Collectors.joining(", "));

    }

}