Add structured logging option

[Steffen911]

What

We would like to configure structured logging for the gtoken tool. I.e. we'd expect log messages to be optionally JSON formatted with a message, a severity, and a timestamp. This could be achieved by replacing log with slog.

Why

In production deployments all messages are currently classified as error given that there is no auxiliary information. Hence, our log alerting consistently triggers on messages from gtoken.