DurationField output format

[sevdog]

Description

As explained in #8527 this PR will provide the ability to choose the output format for DurationFields.

Since Python builtim timedelta object does not have any format function we are going to allow only formats which are already defined in Django codebase in the package django.utils.duration:

• duration_string
• duration_iso_string

[lovelydinosaur]

Thanks - good to talk this through from a docs-first perspective.

[stale[bot]]

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