feat(client): webhook url support on queue submit

[drochetti]

Notes

This PR adds support to specify a webhookUrl to the fal.queue.submit API. It also adds the WebhookResponse<Payload> type so server-side implementations can cast the parsed response to the appropriate type.

const { request_id } = await fal.queue.submit('model-id', {
  webhookUrl: 'https://url.to/my/server/webook/handler'
});

On TypeScript codebases, the WebhookResponse type can help when webhook payloads are received:

import { WebhookResponse } from '@fal-ai/serverless-client';

// on your server you can (or something along those lines)
const result: WebhookResponse = await response.json();

if (result.status === 'OK') {
  // result.error is not available
}