Prototyped several interfaces, added basic topics stuff and exception.

src/Exception/Auth/InvalidAuthCredentials.php

@@ -0,0 +1,11 @@
+<?php
+
+namespace Toloka\PhpApi\Exception\Auth;
+
+use Toloka\PhpApi\Exception\AuthException;
+
+class InvalidAuthCredentials extends AuthException {
+
+  protected $message = 'Failed to login using given credentials.';
+
+}