lib: Add `Identifiable' interface for objects that are identifiable by an ID of any type

library/Icinga/Data/Identifiable.php

@@ -0,0 +1,18 @@
+<?php
+// {{{ICINGA_LICENSE_HEADER}}}
+// {{{ICINGA_LICENSE_HEADER}}}
+
+namespace Icinga\Data;
+
+/**
+ * Interface for objects that are identifiable by an ID of any type
+ */
+interface Identifiable
+{
+    /**
+     * Get the ID associated with this Identifiable object
+     *
+     * @return mixed
+     */
+    public function getId();
+}