MDL-61477 admin: sitepolicy handler API

- Define sitepolicy handler manager class, base class and the core handler
- Allow to set a plugin as sitepolicyhandler that implements the sitepolicy API
- Modify web services to return information from the 3rd party handler instead of core if needed

lib/adminlib.php

@@ -10560,3 +10560,46 @@ class admin_setting_agedigitalconsentmap extends admin_setting_configtextarea {
         return true;
     }
 }
+
+/**
+ * Selection of plugins that can work as site policy handlers
+ *
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @copyright 2018 Marina Glancy
+ */
+class admin_settings_sitepolicy_handler_select extends admin_setting_configselect {
+
+    /**
+     * Constructor
+     * @param string $name unique ascii name, either 'mysetting' for settings that in config, or 'myplugin/mysetting'
+     *        for ones in config_plugins.
+     * @param string $visiblename localised
+     * @param string $description long localised info
+     * @param string $defaultsetting
+     */
+    public function __construct($name, $visiblename, $description, $defaultsetting = '') {
+        parent::__construct($name, $visiblename, $description, $defaultsetting, null);
+    }
+
+    /**
+     * Lazy-load the available choices for the select box
+     */
+    public function load_choices() {
+        if (during_initial_install()) {
+            return false;
+        }
+        if (is_array($this->choices)) {
+            return true;
+        }
+
+        $this->choices = ['' => new lang_string('sitepolicyhandlercore', 'core_admin')];
+        $manager = new \core_privacy\local\sitepolicy\manager();
+        $plugins = $manager->get_all_handlers();
+        foreach ($plugins as $pname => $unused) {
+            $this->choices[$pname] = new lang_string('sitepolicyhandlerplugin', 'core_admin',
+                ['name' => new lang_string('pluginname', $pname), 'component' => $pname]);
+        }
+
+        return true;
+    }
+}