Enrich helm events with user specified attributes to make notifications more informational

[willejs]

Currently, helm release events just contain the helm chart version, which is not particularly useful as in most cases the underlying image tag is changing, but not the chart version. In your deployments slack channel, you will probably see something like this:

What I really want to know is probably which image tag is getting deployed, perhaps I could specify a go template string and use either git commit info from the original control repo, or values in the supplied values.yaml to identify the image tag?