Improve logging support

[khvn26]

Golang's logging ecosystem is vast, and users expect various logging models to be supported. For instance, one of our enterprise customers expects slog support. Currently, the package expects a Logger interface passed as means to log its events. Apparently this is not flexible enough.

Options to consider:

1. Remove logging altogether; return errors where appropriate.
2. Add explicit support for slog by adding a SetHandler method.
3. Simplify expected logger dependency by accepting a logFn function with one msg argument. Have a null logger as default.

[khvn26]

After a chat with a customer we're leaning towards option number 2 as other logging implementations are supposed to be easily wrappable with slog.

We're keeping in mind, however, that the package does very little logging and it's easy to remove completely.

[rolodato]

Resolved in #155, which was released in v4.2.0.