Make listener contracts more clear and enforceable, OR remove them

[georgii-borovinskikh-sonarsource]

We currently have models & interfaces for SLCoreListeners living in the main SLCore assembly, while the implementations live in the SLCore.Listeners assembly.

For example, src/SLCore/Listener/Logger/ILoggerListener.cs vs. src/SLCore.Listeners/Implementation/LoggerListener.cs

The issue is that the ILoggerListener interface is marked as unused, which may confuse people unfamiliar with the project. The initial goal of adding the interfaces was to declare an external contract mainly for self-documentation's sake, but also for possible mock-ing in ITs later.

It needs to be made clear and self-evident what the purpose of those interfaces is, otherwise they need to be removed.