Implement config notification event

Add fieldChanged event to config objects that is triggered when the
value of a field in a group belonging to it is changed.

lib/plugin.js

@@ -42,6 +42,19 @@ class BaseInstancePlugin {
 	 */
 	async init() { }
 
+	/**
+	 * Called when the value of a config field changed.
+	 *
+	 * Invoked after the value of the config field given by `field` has
+	 * changed.
+	 *
+	 * @param {module:lib/config.ConfigGroup} group -
+	 *     The group who's field got changed on.
+	 * @param {string} field - Name of the field that changed.
+	 * @param prev - The previous value of the field.
+	 */
+	async onInstanceConfigFieldChanged(group, field, prev) { }
+
 	/**
 	 * Called before collecting Prometheus metrics
 	 *