Update Session.php (laravel#54421)

Added flash method to Session interface

Author: eldair

src/Illuminate/Contracts/Session/Session.php

@@ -98,6 +98,15 @@ public function pull($key, $default = null);
      */
     public function put($key, $value = null);
 
+    /**
+     * Flash a key / value pair to the session.
+     *
+     * @param  string  $key
+     * @param  mixed  $value
+     * @return void
+     */
+    public function flash(string $key, $value = true);
+
     /**
      * Get the CSRF token value.
      *