Convert map result into `serde_json::Value`

[Martin Junghanns]

It would be nice to be able to convert a Map result into a serde_json::Value. Something like

    while let Some(row) = result.next().await.unwrap() {
        let event: serde_json::Value = row.get("event").unwrap();
        println!("{event:?}");
    }

this currently fails with

called `Result::unwrap()` on an `Err` value: InvalidType { received: Map, expected: "any valid JSON value" }