Allow custom debug logger implementation

[anuraaga]

Summary

Allow a custom DebugLogger implementation

Basic example

type envoyDebugLogger {}

func (l *envoyDebugLogger) Info(message string, args ...interface{}) {
  proxywasm.LogInfof(message, args...)
}

Motivation

We would like debug logs to go to Envoy logs when loading coraza as a wasm filter. Currently DebugLogger is a struct and can only direct logs to log.Logger, it would be nice if it were an interface that can have the current implementation or a custom one. @jptosso does this sound reasonable?

[jptosso]

Merged #337