Add optional support for serde to prost-types

[jo-wil]

Add optional support for serde to prost-types

Happy to open a PR but having opt in serde implementations for timestamp, duration, etc would be awesome.

Let me know if you all are receptive to the idea.

[jo-wil]

I found this https://github.com/xyzzyz/prost-canonical-serde/blob/master/prost-canonical-serde/src/canonical/wkt.rs and it seems like this is the only file that would need to be moved into the main prost-types since prost-types owns Timestamp.