Add the possibility to create external tasks

app/Core/ExternalTask/ExternalTaskInterface.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace Kanboard\Core\ExternalTask;
+
+/**
+ * Interface ExternalTaskInterface
+ *
+ * @package Kanboard\Core\ExternalTask
+ * @author  Frederic Guillot
+ */
+interface ExternalTaskInterface
+{
+    /**
+     * Return Uniform Resource Identifier for the task
+     *
+     * @return string
+     */
+    public function getUri();
+
+    /**
+     * Return a dict to populate the task form
+     *
+     * @return array
+     */
+    public function getFormValues();
+}