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/gateway-main.go

@@ -228,6 +228,12 @@ func StartGateway(ctx *cli.Context, gw Gateway) {
 	globalServerConfig = srvCfg
 	globalServerConfigMu.Unlock()
 
+	// Load logger subsystem
+	loadLoggers()
+
+	// This is only to uniquely identify each gateway deployments.
+	logger.SetDeploymentID(os.Getenv("MINIO_GATEWAY_DEPLOYMENT_ID"))
+
 	var cacheConfig = globalServerConfig.GetCacheConfig()
 	if len(cacheConfig.Drives) > 0 {
 		var err error