log: Roundtrip `log.Logger` type

The log.Logger type still converts to a logrus.Entry when calling functions like WithField() or WithError(). This was hard to change though because a lot of callsites still relied on this conversion. By now we have have fixed all blockers though, which allows us to finally do the conversion and have the log.Logger type roundtrip to itself.

To do so, we inline the logrus.FieldLogger interface and adapt the specific functions to return a Logger instead of a logrus.Entry. This change also demonstrates that all internal interfaces now use the interface exclusively and that we are free to adapt the interface as required from now on.

Note that we also have to add two additional functions to the interface to create the logging interceptors for the gRPC server. This is because we have no way to create these interceptors manually from a log.Logger anymore and thus need its help to do so for us.

The MR also addresses some other cases where we still relied on the logrus package such that overall our reliance on it is significantly reduced now.