add into_json feature

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "axum-valid"
-version = "0.3.0"
+version = "0.4.0"
 description = "Validation tools for axum using the validator library."
 authors = ["GengTeng <me@gteng.org>"]
 license = "MIT"
@@ -23,6 +23,11 @@ edition = "2021"
 axum = { version = "0.6.18", default-features = false }
 validator = "0.16.0"
 
+[dependencies.serde_json]
+package = "serde_json"
+version = "1.0.103"
+optional = true
+
 [dev-dependencies]
 anyhow = "1.0.71"
 axum = { version = "0.6.18" }
@@ -38,3 +43,4 @@ default = ["json", "form", "query"]
 json = ["axum/json"]
 form = ["axum/form"]
 query = ["axum/query"]
+into_json = ["serde_json"]