[FEAT] Add Dynu back to SWAG

[DustyRah]

Is this a new feature request?

• I have searched the existing issues

Wanted change

Support for Dynu was removed from SWAG since the plugin was no longer maintained. We created an updated plugin. Could you please add Dynu back to SWAG?

https://pypi.org/project/certbot-dns-dynudns/

Reason for change

A lot users were using Dynu as part of SWAG but support for Dynu was removed from SWAG since the plugin was not maintained.

Proposed code change

Add Dynu back to SWAG.

[github-actions]

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