chore: add vendor dependencies for kauma build

vendor/serde_json/tests/regression/issue520.rs

@@ -0,0 +1,20 @@
+#![allow(clippy::float_cmp)]
+
+use serde_derive::{Deserialize, Serialize};
+
+#[derive(Serialize, Deserialize, Debug)]
+#[serde(tag = "type", content = "data")]
+enum E {
+    Float(f32),
+}
+
+#[test]
+fn test() {
+    let e = E::Float(159.1);
+    let v = serde_json::to_value(e).unwrap();
+    let e = serde_json::from_value::<E>(v).unwrap();
+
+    match e {
+        E::Float(f) => assert_eq!(f, 159.1),
+    }
+}