Implement responses API streaming

[thekid]

Streaming works differently in the reponses API; and is more comparable to the WebSocket API, sending events with types and payloads, and sending deltas as well as the final values, requiring no extra data merging. This pull request adds an events() method to yield the event types and its JSON values.

Here's an example of using it in conjunction with the responses API:

use com\openai\rest\OpenAIEndpoint;
use util\cmd\Console;

$ai= new OpenAIEndpoint('https://'.getenv('OPENAI_API_KEY').'@api.openai.com/v1');

$events= $ai->api('/responses')->stream([
  'model' => 'gpt-4o',
  'input' => $argv[1],
]);
foreach ($events as $type => $value) {
  if ('response.output_text.delta' === $type) {
    Console::write($value['delta']);
  }
}
Console::writeLine();

See #20 (comment)

[thekid]

The Responses API represents the future direction for building agents on OpenAI.
Source: https://platform.openai.com/docs/guides/responses-vs-chat-completions

To prioritize the responses API, we could use the stream() name for this, and rename the original, which returns "completion chunks" to completions() or chunks().