Feature: support proxy-group in relay (#597)

constant/adapters.go

@@ -69,6 +69,8 @@ type ProxyAdapter interface {
 	SupportUDP() bool
 	MarshalJSON() ([]byte, error)
 	Addr() string
+	// Unwrap extracts the proxy from a proxy-group. It returns nil when nothing to extract.
+	Unwrap(metadata *Metadata) Proxy
 }
 
 type DelayHistory struct {