feat: support requestid for idempotent write ops

[vdor]

What

• DataService::setRequestId($id)
• Appends ?requestid=... to write URIs
• Append only for Add, Update, Void, SendEmail
• After each write, requestid is reset (opt-in per call, prevents accidental reuse)

Why

Adds official idempotency support per Intuit docs:

• https://developer.intuit.com/app/developer/qbo/docs/learn/learn-basic-field-definitions#request-id
• https://help.developer.intuit.com/s/article/What-is-RequestId-and-its-usage

Usage

use QuickBooksOnline\API\DataService\DataService;

$dataService = DataService::Configure($ops);
$result = $dataService->setRequestId('uuid-1234')->Add($invoice);

Closes #563