List webhook subscriptions

Webhook subscriptions deliver patient.created, patient.updated, patient.deleted, appointment.created, appointment.updated, appointment.deleted, invoice.* and payment.* events to your server as signed POST requests (X-Dododentist-Signature: t=<timestamp>,v1=<hex HMAC-SHA256 of "timestamp.body">). An endpoint failing 20 times in a row is disabled automatically. Subscriptions are managed with an operator access token; the secret is only returned once, on create.