feat: support custom tracker url (#59)

pkg/protocol/http/model.go

@@ -0,0 +1,11 @@
+package http
+
+type ReqExtra struct {
+	Method string            `json:"method"`
+	Header map[string]string `json:"header"`
+	Body   string            `json:"body"`
+}
+
+type OptsExtra struct {
+	Connections int `json:"connections"`
+}