Logging: print `message` as JSON, instead of wrapping into a string

[mriccia]

Is your feature request related to a problem? Please describe.

The Powertools Logging functionality automatically wraps the messages into string, regardless of whether they are strings or objects.
Looking through the logs with CloudWatch logs is harder with the message being printed as a single string, and printing as JSON will also unlock other options (e.g. using CloudWatch insights to automatically discover fields).

Describe the solution you'd like

Describe alternatives you've considered

Additional context

[jeromevdl]

I think this is a breaking change ==> v2

[scottgerring]

This feels quite important. Is it really breaking and not a bug?
OTOH we are redoing the logging in v2 anyway, so there's a natural synergy there.

[jeromevdl]

@jdoherty I'm taking this one. I'm working on the logging v2 anyway. Not sure that make sense you spend time on a pure log4j solution as I'm using slf4j now.

[jeromevdl]

https://stackoverflow.com/questions/77543791/slf4j-and-log4j2-json-messages-escaped