feature #19291 [HttpClient] Add HttpOptions::addHeader method (Dean151)

This PR was squashed before being merged into the 7.1 branch.

Discussion
----------

[HttpClient] Add `HttpOptions::addHeader` method

Fixes #19236

Not sure of the documentation format. Open to suggestion to improve it :)

Commits
-------

436f69e [HttpClient] Add `HttpOptions::addHeader` method

Author: OskarStark

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`_