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

binding/json.go

@@ -10,7 +10,7 @@ import (
 	"io"
 	"net/http"
 
-	"github.com/gin-gonic/gin/internal/json"
+	"github.com/gin-gonic/gin/codec/json"
 )
 
 // EnableDecoderUseNumber is used to call the UseNumber method on the JSON
@@ -42,7 +42,7 @@ func (jsonBinding) BindBody(body []byte, obj any) error {
 }
 
 func decodeJSON(r io.Reader, obj any) error {
-	decoder := json.NewDecoder(r)
+	decoder := json.API.NewDecoder(r)
 	if EnableDecoderUseNumber {
 		decoder.UseNumber()
 	}