Add webhook for when approaching the usage limit

[jdolle]

When hitting a configured breakpoint for usage, e.g. 80%/90%/95% of a subscription's usage limit, fire a webhook.

This should not spam if usage is teetering at the limit. So a reasonable delay for re-notifying should be set. This notification may be sent via email, slack, or potentially other APIs.

[jdolle]

Customers have requested integrations like slack, email, or webhook. Also the ability to customize a percentage for critical and warnings with different notification channels