Expose in-flight Request via optional 'request' option callback

[owner888]

What

Expose the in-flight Request object to callers via an optional 'request' option callback:

$http->request($url, [
    'method' => 'POST',
    'request' => function ($request) use (&$handle) { $handle = $request; },
    // ...
]);
// later, e.g. when the downstream client disconnects:
$handle->getConnection()->close();

Why

There is currently no way to cancel an in-flight request. For long-running streaming
responses (e.g. LLM SSE streaming via webman/openai), when the downstream client
disconnects, the upstream request keeps running to completion — wasting resources
(and paid LLM tokens).

With the Request handle exposed, callers can close the underlying connection to abort.

Notes

• Fully backward compatible: nothing changes unless the 'request' option is set.
• When a redirect is followed, the callback fires again with the new Request,
  so the caller always holds the live handle.