feat(api): add webhook endpoint management (#1020)

## Summary

Adds webhook endpoint management and event-type discovery through the
webhooks resource.

## Changes

- Create, list, retrieve, update, and delete webhook endpoints
- Rotate signing secrets and send sample events
- List the webhook event types available to the project

Co-authored-by: Castiron Bot <castiron@openai.com>

Author: jbeckwith-oai

.castiron.stats.yml

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