Support shared secrets for webhook authentication

[lizziegooding]

Context

Adds support for shared secrets to webhooks so webhook receivers can authenticate requests.

Summary of changes

• [x] add a new secret column to webhooks table where we store secrets
• [x] add the option for users to specify their own secret, otherwise generate a random string
• [x] return the secret on create; don't return the secret on all other webhook requests
• [x] add url validation to the user passed url on create and update
• [x] have webhooks send a presigned url with the hmac sha256 hashed payload and shared secret sent as a query parameter named signature

Next steps

[punt] write tests for the send function that sends webhooks

• [ ] support secrets in hecate UI
• [ ] test on staging
• [ ] review
• [ ] merge & release

cc/ @ingalls @miccolis @samely @mattciferri

[lizziegooding]

UI updates: adds a webhook secret form on create

Shows the user a modal on successful create with the webhook secret Hecate responds with