spec: Snapshot.created_at should use well-known Timestamp protobuf

[jdef]

https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/timestamp.proto

This would be a breaking change after 1.0, so if this change doesn't land before then maybe we could eventually use a oneof to wrap the existing int-based timestamp and a new field that uses Timestamp.

Ideally we'd resolve this prior to 1.0

[saad-ali]

I support this change.