restore missing serde_json dev-dependency

erroneously removed in 78589f5

Cargo.toml

@@ -46,6 +46,7 @@ thiserror = "1.0"
 criterion = "0.5"
 csv = "1"
 mock_instant = { version = "0.5" }
+serde_json = "1.0"
 # By default, reqwest builds openssl from source, which fails on missing/incompatible system dependencies
 reqwest = { version = "0.11", features = ["rustls-tls"], default-features = false }
 futures = "0.3"