Add audit logging for S3 and Web handlers (#6571)

This PR brings an additional logger implementation
called AuditLog which logs to http targets

The intention is to use AuditLog to log all incoming
requests, this is used as a mechanism by external log
collection entities for processing Minio requests.

cmd/logger/targets.go

@@ -20,7 +20,7 @@ package logger
 // a single log entry and send it to the log target
 //   e.g. send the log to a http server
 type LoggingTarget interface {
-	send(entry logEntry) error
+	send(entry interface{}) error
 }
 
 // Targets is the set of enabled loggers