Overhaul [1/n]

Signed-off-by: Sean Du <do4suki@gmail.com>

internal/handler/api/payment/payment.go

@@ -0,0 +1,13 @@
+package payment
+
+import "fmt"
+
+func PaymentApi(action string) (res any, err error) {
+	switch action {
+	case "productList":
+		res, err = productList()
+	default:
+		err = fmt.Errorf("unimplemented action: payment: %s", action)
+	}
+	return res, err
+}