Remove `prost_types::Timestamp` from the public API

[ielis]

It would be better not to have prost_types::Timestamp in the builder's public API. This way, we can prevent having to import the prost_types crate into user code and we will reduce potential for breaking changes.

Instead of taking a Timestamp, the methods should allow working with more primitive types, such as ISO8601 timestamps, or seconds + nanos, which Timestamp ultimately corresponds to.