Need to be able to include Config fallback values to string representation

[IgorFedchenko]

Basically as stated at akkadotnet/akka.net#3886 - when we use config = config.WithFallback(fallback), and then call config.ToString(), we do not get fallback values included.

This is actually fine by default, but we need to have ability to include fallback values into string representation. Need something like config.ToString(true).