Rotate a webhook signing secret

Issues a new signing secret. The previous secret keeps signing deliveries for 24 hours so receivers can roll over without downtime.

curl -X POST "https://example.com/webhooks/string/rotate-secret"

{  "id": "string",  "url": "string",  "events": [    "member.joined"  ],  "enabled": true,  "createdAt": "2019-08-24T14:15:22Z",  "secret": "string"}