[HttpClient] Add HttpOptions::addHeader method

Author: Dean151

http_client.rst

@@ -150,10 +150,17 @@ brings most of the available options with type-hinted getters and setters::
     $this->client = $client->withOptions(
         (new HttpOptions())
             ->setBaseUri('https://...')
+            // setHeaders() replaces *all* headers at once, and deletes the headers you do not provide
             ->setHeaders(['header-name' => 'header-value'])
+            // add or replace a single header using addHeader()
+            ->addHeader('another-header-name', 'another-header-value')
             ->toArray()
     );
 
+.. versionadded:: 7.1
+
+    The :method:`Symfony\\Component\\HttpClient\\HttpOptions::addHeader` method was introduced in Symfony 7.1.
+
 Some options are described in this guide:
 
 * `Authentication`_