feat: add log/slog for logging

[muhlemmer]

As a package user I want to be able to log errors and (debug) information. x/exp/slog provides a new standard logging API for Go. From go 1.21 it is already available from the standard library. However, we will use the experimental version first, so that we can still build for 1.20.

Original proposal.

Acceptance criteria:

• op: attach the logger to the request context in a middleware
• op: log errors in the error handlers, with logger from context.
• rp: log errors in the error handlers, with logger from context.
• provide examples on how to configure the logger.

The purpose of this issue is to set-up a logger framework for at least the most critical code paths and errors and is mostly related to #291. Later more logging points can be introduced where needed for debug or information.

[livio-a]

was implemented with #432