dns/ddclient: add native bunny.net Dynamic DNS provider

• I have read the contributing guide lines at https://github.com/opnsense/plugins/blob/master/CONTRIBUTING.md
• I have searched the existing issues, open and closed, and I'm convinced that mine is new.
• When the request is meant for an existing plugin, I've added its name to the title.

Is your feature request related to a problem? Please describe.

The native backend of the Dynamic DNS plugin does not support updating DNS records hosted by bunny.net. Users currently need an external script or must update records manually when their public IP address changes.

Describe the solution you'd like

Add a native bunny.net provider to dns/ddclient that looks up the DNS zone by its domain and updates the relevant A/AAAA records. Bunny.net has an OpenAPI spec for its API: https://core-api-public-docs.b-cdn.net/docs/v3/public.json

The current UI fields map cleanly:

• Zone: bunny.net DNS zone domain
• Password: bunny.net API access key
• Hostname(s): full FQDNs to update

Describe alternatives you've considered

Tried the existing custom/custom POST: it doesn't support passing the API key via request headers, or passing a dynamic JSON payload with the new IP.

Additional context

API docs: https://core-api-public-docs.b-cdn.net/docs/v3/public.json

Start by reading the native backend under dns/ddclient and the linked Bunny.net OpenAPI specification. Trace how existing providers resolve zones and update records, then add the Bunny.net provider using the current Zone, Password, and Hostname(s) fields. Done means the provider can look up the zone and update the requested A/AAAA records through the API.