Add support for RFC3339 durations

[wizardishungry]

https://spec.openapis.org/registry/format/duration

There are libraries such as https://github.com/peterhellberg/duration that support parsing RFC3339 durations, however it does not support marshaling time.Duration to a string.

[mromaszewicz]

I'm going to add our own Duration type, which stores the days, months, years, etc, on a structure. https://github.com/peterhellberg/duration makes assumptions about the number of days in a month, which will definitely be wrong for some people, so the only thing that I can do is faithfully store the nidividual time increments, and let the user decide how to turn them into a time.Duration.