webhook support

[zeke]

I would like to be able to specify a webhook URL and webhook_events_filter when creating predictions with the predict() or createPrediction() methods.

Based on the current API, maybe that could work something like this:

const prediction = await replicate
  .model("owner/model:version")
  .createPrediction(inputs, {webhook, webhook_eventsFilter});

// ☝🏼 Don't actually know if this is the signature we want
// Just want to illustrate that we need to pass more than `inputs`.

cc @erbridge @gr2m

[zeke]

This is blocking replicate/scribble-diffusion#26