feat: add self-extractor for JSON type (#4123)

sqlx-core/src/types/json.rs

@@ -86,6 +86,13 @@ use crate::types::Type;
 #[serde(transparent)]
 pub struct Json<T: ?Sized>(pub T);
 
+impl<T> Json<T> {
+    /// Extract the inner value.
+    pub fn into_inner(self) -> T {
+        self.0
+    }
+}
+
 impl<T> From<T> for Json<T> {
     fn from(value: T) -> Self {
         Self(value)