Chore: improve code architecture

adapters/outbound/direct.go

@@ -1,6 +1,7 @@
 package adapters
 
 import (
+	"encoding/json"
 	"net"
 
 	C "github.com/Dreamacro/clash/constant"
@@ -40,6 +41,12 @@ func (d *Direct) Generator(metadata *C.Metadata) (adapter C.ProxyAdapter, err er
 	return &DirectAdapter{conn: c}, nil
 }
 
+func (d *Direct) MarshalJSON() ([]byte, error) {
+	return json.Marshal(map[string]string{
+		"type": d.Type().String(),
+	})
+}
+
 func NewDirect() *Direct {
 	return &Direct{}
 }