Add utility functions to parse and format GEP-2257 Duration strings for Gateway API

[kflynn]

Signed-off-by: Flynn emissary@flynn.kodachi.com

/kind feature

This PR adds utility functions to parse and format GEP-2257 Durations for Gateway API, as well as unit tests for them. As Gateway API becomes more common, it's nice to be able to work with its resources in all the languages that people use for wrangling Kubernetes.

Add utility functions kubernetes.utils.duration.parse_duration and kubernetes.utils.duration.format_duration to manage Gateway API Duration strings as specified by GEP-2257.

- [GEP] https://gateway-api.sigs.k8s.io/geps/gep-2257/

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[yliaog]

/lgtm
/approve

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[hswong3i]

To downstream OS packager:

This PR newly add a dependency with durationpy>=0.7 (see #2261), which you may not find in any standard Linux distro (see https://pkgs.org/search/?q=durationpy).

Don't forget also package your python3-durationpy as my OBS repo (see https://build.opensuse.org/package/show/home:alvistack/icholy-durationpy-0.9), else you may face runtime error as me, too ;-)