Native support for Serializing/Deserializing prost_types::Timestamp & std::time::SystemTime

[markuskobler]

I realize chrono is currently neatly integrated with firestore-rs but would there be a downside to also supporting google.protobuf.Timestamp and std::time::SystemTime when it comes to serializing and deserializing timestamps?

Would be happy to pull a PR together if you were open to this and had a preferences on how it would be implemented.

Also as an aside we have tried to stick with time instead of chrono so ideally chrono would be a default feature we could disable.