Add webhook callback to auth helper libraries

[scotttrinh]

We can validate the shape, verify the signature (if present), and present the developer with a nice API like:

type WebhookEvent =
  | EmailVerificationRequestedEvent
  | PasswordResetRequestedEvent;

// in `createAuthRouteHandlers`
async onWebhook(data: WebhookEvent) {
  switch (data.event_type) {
    case "EmailVerificationRequested": {
      await emails.sendVerification(data.email, data.verification_token);
      return;
    }
    case "PasswordResetRequested": {
      await emails.sendPasswordReset(data.email, data.reset_token);
      return;
    }
}

For Next.js we could even automatically wrap this in after so it doesn't block sending the webhook response.