Generic Webhook Option for Alerts

[MikeShi42]

Since most of our alert channels today depend on an Oauth-based flow in Cloud, we don't have many alert options available in the OSS version. Additionally even in the cloud, you're limited by the current set of alert integrations we've written today.

We should allow users to configure arbitrary webhook destinations with custom payloads so that they can alert via webhook to other platforms like Pagerduty, Twilio, Sendgrid, etc.