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

src/policy/src/main/java/eu/teraflow/policy/common/Util.java

+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(", "));
+    }
+}