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

go.mod

@@ -9,6 +9,7 @@ require (
 	github.com/goccy/go-json v0.10.2
 	github.com/json-iterator/go v1.1.12
 	github.com/mattn/go-isatty v0.0.20
+	github.com/modern-go/reflect2 v1.0.2
 	github.com/pelletier/go-toml/v2 v2.2.4
 	github.com/quic-go/quic-go v0.52.0
 	github.com/stretchr/testify v1.10.0
@@ -30,7 +31,6 @@ require (
 	github.com/klauspost/cpuid/v2 v2.0.9 // indirect
 	github.com/leodido/go-urn v1.4.0 // indirect
 	github.com/modern-go/concurrent v0.0.0-20180228061459-e0a39a4cb421 // indirect
-	github.com/modern-go/reflect2 v1.0.2 // indirect
 	github.com/onsi/ginkgo/v2 v2.9.5 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/quic-go/qpack v0.5.1 // indirect