support container registry notifications as webhook type in hosted gitlab.com

Problem to solve

Allow users of hosted gitlab.com to configure registry notifications

Further details

configuration is well documented for OSS/EE version: https://docs.gitlab.com/ee/administration/packages/container_registry.html#configure-container-registry-notifications

Proposal

Add support for configuring this notification in the same area as webhooks. This is not to propose details of the design, just a top level feature request since I could not find any pre-existing for this.