Chore: Imperative request data binding (#39837)

* rename Bind to BindMiddleware

* make things private

* removed unused part of data bindings

* provide json and form binding helpers

* add example of binding migration in login api

* implement validation

* fix tests

* remove debug output

* put new bind api into macaron pacakge

* revert bind api breaking change

pkg/macaron/binding.go

@@ -0,0 +1,74 @@
+package macaron
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"reflect"
+)
+
+// Bind deserializes JSON payload from the request
+func Bind(req *http.Request, v interface{}) error {
+	if req.Body != nil {
+		defer req.Body.Close()
+		err := json.NewDecoder(req.Body).Decode(v)
+		if err != nil && err != io.EOF {
+			return err
+		}
+	}
+	return validate(v)
+}
+
+type Validator interface {
+	Validate() error
+}
+
+func validate(obj interface{}) error {
+	// If type has a Validate() method - use that
+	if validator, ok := obj.(Validator); ok {
+		return validator.Validate()
+	}
+	// Otherwise, use relfection to match `binding:"Required"` struct field tags.
+	// Resolve all pointers and interfaces, until we get a concrete type.
+	t := reflect.TypeOf(obj)
+	v := reflect.ValueOf(obj)
+	for v.Kind() == reflect.Interface || v.Kind() == reflect.Ptr {
+		t = t.Elem()
+		v = v.Elem()
+	}
+	switch v.Kind() {
+	// For arrays and slices - iterate over each element and validate it recursively
+	case reflect.Slice, reflect.Array:
+		for i := 0; i < v.Len(); i++ {
+			e := v.Index(i).Interface()
+			if err := validate(e); err != nil {
+				return err
+			}
+		}
+	// For structs - iterate over each field, check for the "Required" constraint (Macaron legacy), then validate it recursively
+	case reflect.Struct:
+		for i := 0; i < v.NumField(); i++ {
+			field := t.Field(i)
+			value := v.Field(i)
+			rule := field.Tag.Get("binding")
+			if !value.CanInterface() {
+				continue
+			}
+			if rule == "Required" {
+				zero := reflect.Zero(field.Type).Interface()
+				if value.Kind() == reflect.Slice {
+					if value.Len() == 0 {
+						return fmt.Errorf("required slice %s must not be empty", field.Name)
+					}
+				} else if reflect.DeepEqual(zero, value.Interface()) {
+					return fmt.Errorf("required value %s must not be empty", field.Name)
+				}
+			}
+			if err := validate(value.Interface()); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}