Support conversions to/from datatypes in jiff time handling library

jiff is a promising time (and timezone) handling crate based on the design of the Temporal JavaScript API. It is planned for a long-term 1.0 version to be released later in the year, see https://github.com/BurntSushi/jiff/releases/tag/0.2.0

I expect it to be a very important part of the library ecosystem, so it would be nice to get basic support for it, at least with relevant `From`/`TryFrom` implementations. The types I'm thinking of here are `google_cloud_wkt::Duration` and `google_cloud_wkt::Timestamp`. Due to rules on where trait implementations can be placed, probably the easiest place is to add them in this repository under a feature flag.