dns/ddclient: add porkbun support to native backend

[5t0n3]

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• I have read the contributing guidelines at https://github.com/opnsense/plugins/blob/master/CONTRIBUTING.md
• I opened an issue first for non-trivial changes and linked it below.
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Describe the problem

The existing ddclient-based Porkbun dynamic DNS backend only supports setting subdomain records (e.g., subdomain.example.com but not example.com) due to the on-root-domain option not being exposed in the OPNSense UI. It sounds like ddclient may be removed from OPNsense soon as well (see e.g. opnsense/ports#223 (comment)), at which point OPNsense would not support Porkbun DNS at all.

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Describe the proposed solution

This PR adds a Python implementation to the native dynamic dns backend to support Porkbun's API. I modeled it after the Cloudflare backend script with changes to suit the different API structure.

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Related issue

#4761 (mentions non-root-domain breakage)