feat: support custom json codec at runtime (#3391)

* refactor(json): export json codec

* feat(json): support custom json codec at runtime

* chore(copyright): update copyright to 2025 gin core team

* docs(gin): add custom json codec examples in doc file

codec/json/json.go

@@ -0,0 +1,41 @@
+// Copyright 2025 Gin Core Team. All rights reserved.
+// Use of this source code is governed by a MIT style
+// license that can be found in the LICENSE file.
+
+//go:build !jsoniter && !go_json && !(sonic && (linux || windows || darwin))
+
+package json
+
+import (
+	"encoding/json"
+	"io"
+)
+
+// Package indicates what library is being used for JSON encoding.
+const Package = "encoding/json"
+
+func init() {
+	API = jsonApi{}
+}
+
+type jsonApi struct{}
+
+func (j jsonApi) Marshal(v any) ([]byte, error) {
+	return json.Marshal(v)
+}
+
+func (j jsonApi) Unmarshal(data []byte, v any) error {
+	return json.Unmarshal(data, v)
+}
+
+func (j jsonApi) MarshalIndent(v any, prefix, indent string) ([]byte, error) {
+	return json.MarshalIndent(v, prefix, indent)
+}
+
+func (j jsonApi) NewEncoder(writer io.Writer) Encoder {
+	return json.NewEncoder(writer)
+}
+
+func (j jsonApi) NewDecoder(reader io.Reader) Decoder {
+	return json.NewDecoder(reader)
+}