[HttpClient] Add JsonMockResponse

Author: alamirault

http_client.rst

@@ -2032,6 +2032,19 @@ Then configure Symfony to use your callback:
             ;
         };
 
+You can also use :class:`Symfony\\Component\\HttpClient\\Response\\JsonMockResponse` which automatically encode
+and set `application/json` header::
+
+    use Symfony\Component\HttpClient\Response\JsonMockResponse;
+
+    $response = new JsonMockResponse([
+        'foo' => 'bar',
+    ]);
+
+.. versionadded:: 6.3
+
+    The ``JsonMockResponse`` was introduced in Symfony 6.3.
+
 Testing Request Data
 ~~~~~~~~~~~~~~~~~~~~