Do not depend on concrete logger

[luhring]

The Melange Pipeline type shouldn't rely on a concrete type like *logrus.Entry, because this requires users to have a concrete type or nil, and nil causes panics in the logger implementation.

Using an abstraction instead allows consumers to use other loggers (including a nopLogger) when logging isn't needed or wanted.

This caused a panic in wolfictl, and a forthcoming wolfictl PR will take advantage of this change (with tests!).