Agree with @remie - this is not great, especially having logic to handle binary formats, for payloads that a re <2kb? Why not just use normal webhooks.
Also, what are we supposed to do with ARIs? We currently only work with projectIds/projectKeys, since the REST APIs only work with those, how are we supposed to map those ids to the one we store on our side? I understand that internally GraphQL is the way forward, but almost no one, besides a few Forge apps, will be able to work with this.