feat: add AfterOffset for testing stall control

[bshaffer]

Adds after_offset config to chunk upload in order to test stall control.

See https://github.com/googleapis/google-cloud-php/blob/3cda3dc6ac4af05d508794c4a8eaabd3bd9e3068/Gax/tests/Conformance/ResumableUploadTest.php#L212-L232 for an example:

        $chunkSize = 256 * 1024;
        $payload = str_repeat('c', $chunkSize * 2);
        $headers = [
            'X-Goog-Test-Scenario-Config' => json_encode([
                'delay_ms' => 1500,
                'after_offset' => $chunkSize,
            ])
        ];
        $resumableUploadOptions = [
            'chunkSize' => $chunkSize,
            'transferStallMinimumRate' => 10,
            'transferStallTimeout' => 1,
        ];

        $this->expectException(ApiException::class);
        $this->expectExceptionMessage('Upload stalled.');

        $this->createClientAndUpload($payload, null, $headers, $resumableUploadOptions);

[viacheslav-rostovtsev]

LGTM