Give Zeno the logging it deserves

[equals215]

Zeno logging is chaotic and need to be standardized.
The current standard for logging is slog as it was recently introduced in the Go standard library.

• multiHandler implements slog.Handler interface with support for multiple outputs, currently fixed with stdout and file output
• file logging is done in json and stdout logging is in text format, both outputs' logging level can be configured

TODO :

• replicate utils.SetupLogging() behaviour in log
  • ElasticSearch logging should be handled by log
  • Index rotation every day

[equals215]

yeah it works, elasticsearch log forwarder with slog implementation (sick huh?)

[equals215]

@CorentinB your turn!

[equals215]

still need to add the capacity to specify log file path, will do in the upcoming CLI rewrite PR

[CorentinB]

Looks VERY good to me. Once the few small changes requested are done, it's good to merge. Thanks!

[equals215]

done