Change pipeline hook type to plain func instead of interface

[ccremer]

Summary

• Listener is not an interface with Accept(step) anymore, but func(step).

This makes anonymous hooks easier to implement

Checklist

• Categorize the PR by setting a good title and adding one of the labels:
  fix, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog
• Update documentation.
• Update tests.