feat(Gax): support chunkSize on resume and surface actual chunk size

[bshaffer]

Updates Resumable Uploads to allow specifying a chunkSize when resuming an upload, and exposes getter/setter methods to access the actual/negotiated chunk size.

1. Pass chunkSize on resume: Users can now pass chunkSize in $optionalArgs to resumeUpload() (or via startResumableUploadCall), or pass it directly into $upload->startUpload($stream, ['chunkSize' => ...]).
2. Access the actual chunk size: Added getChunkSize(): ?int and setChunkSize(int $chunkSize): void on ResumableUpload. If the server responds with a chunk granularity (X-Goog-Upload-Chunk-Granularity), the chunk size is adjusted down to the closest multiple and surfaced on the upload object.
3. Resuming documentation: Added docblock notes recommending that streams are rewound to offset 0 before resuming, so unconfirmed chunks can be re-read if needed.

Example usage:

// 1. Specify chunk size when resuming
$resumableUpload = $client->resumeUpload($uploadUrl, 'createUpload', [
    'chunkSize' => 5 * 1024 * 1024, // 5 MB chunks
]);

// 2. Or retrieve the actual chunk size used/negotiated
$actualChunkSize = $resumableUpload->getChunkSize();

// 3. Start/resume upload
$response = $resumableUpload->startUpload($dataStream);

[viacheslav-rostovtsev]

SGTM!