Description
Usually you add a header like this - see https://symfony.com/doc/current/http_client.html#headers

$response = $client->request('POST', 'https://...', [
    'headers' => [
        'User-Agent' => 'foo',
    ],
]);

But how can you add a header when doing a file upload? https://symfony.com/doc/current/http_client.html#uploading-data doesn't say anything about this, so I'm guessing:

$headers = $formData->getPreparedHeaders()->toArray();
$headers[] = 'User-Agent: foo';

Questions:

1. Is this really the best way to do it? => If yes, I'd come up with a PR to add this to the docs
2. Notice the inconsistent syntax: associative array vs. indexed array. So what about a addHeader method that works in all cases?
3. (somewhat off-topic) It would be nice to have a method to override the config from framework.yaml for the client (not the request). My use case: I'm doing $client-request() multiple times, and I need the same User-Agent for all of them. The string is dynamic (so I can't set it in framework.yaml), but it's the same for every PHP session, so something like $client->setOptions(...) would work.