Update cache using Github webhooks

[umglurf]

As either a supplement to the regular cache refresh or a replacement, it would improve the registry by listening to Github webhooks and refresh when releases has been done.

[stigok]

Useful resources

• https://docs.github.com/en/developers/webhooks-and-events/webhooks/webhook-events-and-payloads#release
• https://github.com/cbrgm/githubevents
• https://docs.github.com/en/rest/webhooks?apiVersion=2022-11-28#pubsubhubbub

[stigok]

This needs to be thought through better for cases where we are running multiple replicas behind a load balancer, as is the case when we run this in Kubernetes. Only one of the replicas will receive the event, and this goes into the same bucket of problems related to leader election and replication across replicas.