Verify Mailgun webhooks

# read from a no-echo prompt (or piped stdin) — never an argv flag
printf %s "$MAILGUN_SIGNING_SECRET" | wbhk endpoints add-provider-secret <endpoint-id> \
  --provider mailgun

curl https://api.webhook.co/v1/endpoints/<endpoint-id>/provider-secrets \
  -H "Authorization: Bearer $WEBHOOK_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"provider":"mailgun","secret":"<your-value>"}'

await webhook.endpoints.providerSecrets.add({
  endpointId,
  provider: "mailgun",
  secret: process.env.MAILGUN_SIGNING_SECRET!,
});