Hi guys, I'm trying to use Event Webhooks, in order to sync my experiments with the back-end. The issue is I couldn't find the key (secret) to verify the payload using signature header 'x-growthbook-signature', did the verification method change?, or is it for paid version only? the SDK Webhook Integration works fine since I can find the secret, but It doesn't fire when an experiment change so it's not a suitable for my use case.