Add user backend provider system

app/Core/User/UserBackendProviderInterface.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Kanboard\Core\User;
+
+/**
+ * User Backend Provider Interface
+ *
+ * @package  Kanboard\Core\User
+ * @author   Frederic Guillot
+ */
+interface UserBackendProviderInterface
+{
+    /**
+     * Find a user from a search query
+     *
+     * @access public
+     * @param  string $input
+     * @return UserProviderInterface[]
+     */
+    public function find($input);
+}