Add implementation of CandidType for serde_json::Value

[lastmjs]

See this forum post for more information: https://forum.dfinity.org/t/derive-candidtype-with-serde-json/5800

Seems it would be nice for candid to implement CandidType on serde_json::Value. This would allow for candid serialization of arbitrary JSON values. I would implement CandidType myself, but in my crates I am depending on serde_json::Value and candid directly, thus I can't implement CandidType without relying on some kind of wrapper around serde_json::Value. Seems serde_json::Value is the canonical way to serialize and deserialize arbitrary JSON in Rust, so could it be added to the library here? https://github.com/dfinity/candid/blob/master/rust/candid/src/types/impls.rs

I am happy to do the work if it would be accepted.