log: Wrap the logrus logger

Introduce a wrapper for the logrus logging infrastructure. This is a first step towards abstracting away the actual implementation of our logs behind a type that we can iterate on and will thus pave the way towards adoption of the slog package.

Note that this is only one step of this transition. The interface does not yet roundtrip to itself, e.g. calling log.Logger.WithError() will result in a logrus.Entry, not in a log.Logger. This will be handled at a later point.