Publish webhook event for grant is revoked

[mkurapov]

Context

A requirement from JOPACC (and sensible for all integrators), we need to publish a webhook for when a grant is revoked, such that an integrator, for example, can release funds on hold for a particular user account. This will be the first event that the auth service publishes, so we will need to scaffold the webhook workers in that package as well (publishing to a new ENV variable, WEBHOOK_URL). For now, we just need to publish the grantId in the payload, as well as the time it was revoked.

It is recommended to do this in multiple parts.