[WEBHOOK] Allow an optional callback_url parameter on the enqueue call, as an alternative to webhook_ids

Today webhooks must be pre-registered per model in the platform UI, and the enqueue call only accepts webhook IDs (webhook_ids). There is no way to give a URL at request time.

This does not scale well for us:

• We have N models and several environments (local, dev, staging, prod), so we have to create and maintain N x M endpoints by hand in the UI, plus one per developer for local work.

• Each environment then needs its own MINDEE_V2_WEBHOOK_ID value to keep in sync, so configuration lives half in your UI and half in our code.

• For local development the tunnel URL (ngrok, localtunnel) changes regularly, so a developer has to go back into the UI and edit the endpoint every time.