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)

[austin2118ace]

Ah finally big fan of this!

[jawereq]

Hi @5t0n3, @AdSchellevis, and others.

I have been running a modified version of this PR in my router for the last month or so, and it has been successfully updating both my IPv4 and IPv6 addresses for my Porkbun domain. This is something I couldn't make work with the legacy ddclient. I tried to handle all the edge cases. The script first tries updating records, which works if they exist. If they don't, it tries to create them.

I would love to get these changes integrated to mainline. My current PR is targeting your fork, @5t0n3. I didn't want to open a duplicate PR to the main plugins project, but I can if this would be better. My current PR is the one above, 5t0n3#1.

[AdSchellevis]

merged, thanks!