[FEATURE] Reevaluate Logging framework and maybe switch

[aepfli]

Requirements

Currently, flagd is using ZAP as a logging framework, which works great. ZAP was chosen because no logging framework was delivered with Go. Now, there is slog, which offers all the functionality needed but is a standard library package.

One of the reasons for proposing this change is open-feature/go-sdk-contrib#522 and open-feature/go-sdk#260. To enable better interoperability, it would make sense to evaluate and maybe switch to slog.

[djosephsen]

I'll probably take a crack at this together with a co-worker who is interested in getting into contributor-ing if nobody else wants it. We'll likely begin by refactoring the current zap wrapper into a generic Logger interface, keeping the zap logger, and adding a new implementation for slog.. at that point it should be pretty clear which makes more sense and the switch should be trivial either way

[beeme1mr]

Sure, that would be great. Thanks @djosephsen!