Add logging facilities

[tmadlener]

Currently podio logs directly to stdout or stderr in a limited number of places. It would be nice if these messages could be picked up (and silenced depending on level) by downstream consumers.

This will require at least some logging facilities that make it possible to do so. One possible approach is to pass a logger object to all functions or object that need to do logging (defaulted to some default logger that preserves current behavior).

[aashirvad08]

I am slightly cautious about "pass a logger everywhere" as the primary model as with relatively few logging sites in PODIO, that risks lot of plumbing for limited gain
A process-wide hook set at reader/writer construction might achieve the same downstream flexibility without propagating logger ownership throughout the API

std::function<void(Level, std::string_view)>sink feels like a natural fit to me btw

and like

• do level checks need to happen before message construction? zero-overhead disabled paths would require lazy evaluation
• what are the expected lifetime & thread-safety guarantees for the sink --set-once before I/O, or safe to swap mid-run?

Happy to work on this if useful

[tmadlener]

It's a bit more complicated than it might seem at first. The goal is not only to have some more advanced logging for podio, but also to enable hooking it up to an external logging system.

Somewhat similar to what we do with Acts and Gaudi, here:
https://github.com/key4hep/k4ActsTracking/blob/main/k4ActsTracking/include/k4ActsTracking/ActsGaudiLogger.h
https://github.com/key4hep/k4ActsTracking/blob/main/k4ActsTracking/src/ActsGaudiLogger.cpp

Given that this touches at least one downstream framework this will require a bit more work than I think is realistically feasible for an outside contribution at this point.