[Agent][Platform][OpenAI] Add stream usage support

[oesteve]

Q	A
Bug fix?	no
New feature?	yes
Docs?	no
Issues	Fix #753
License	MIT

$options = [
    'stream' => true,
    'stream_options' => [
        'include_usage' => true,
    ],
];

$stream = $agent->call($messages, $options)->getContent();

if (!is_iterable($stream)) {
    throw new \RuntimeException('Invalid result type, expected iterable');
}

foreach ($stream as $chunk) {
    if (is_string($chunk)) {
        $context->setDelta($chunk);
        continue;
    }

    if ($chunk instanceof TokenUsage) {
        $context->addUsedTokens(
            $chunk->totalTokens ?: 0,
            $chunk->cachedTokens ?: 0
        );
    }
}