Markus Kobler

I realize [chrono](https://crates.io/crates/chrono) is currently neatly integrated with `firestore-rs` but would there be a downside to also supporting [google.protobuf.Timestamp](https://docs.rs/prost-types/latest/prost_types/struct.Timestamp.html) and [std::time::SystemTime](https://doc.rust-lang.org/std/time/struct.SystemTime.html) when it comes to serializing and deserializing timestamps? Would...