Add method to get all registered extensions

Author: tobyzerner

src/JsonApi.php

@@ -62,6 +62,14 @@ public function extension(Extension $extension)
         $this->extensions[$extension->uri()] = $extension;
     }
 
+    /**
+     * Get all registered extensions.
+     */
+    public function getExtensions(): array
+    {
+        return $this->extensions;
+    }
+
     /**
      * Define a new resource type.
      */