Expose log.With func

[alexshtin]

Expose log.With helper func which was hidden behind internal before. Simple func call won't work here because it makes package cycle, therefore I moved entire withLogger to external log package (it is still unexported though).

With this change users will be able to do something like this in workflow code:

logger := log.With(workflow.GetLogger(ctx), "customKeyName", "customKeyValue")
logger.Info("message")

same as we do internally.