add unix/serial dialer and connector

dialer/serial/metadata.go

@@ -0,0 +1,12 @@
+package serial
+
+import (
+	md "github.com/go-gost/core/metadata"
+)
+
+type metadata struct {
+}
+
+func (d *serialDialer) parseMetadata(md md.Metadata) (err error) {
+	return
+}