ddns-scripts: add support for Porkbun

ddns-scripts: add support for Porkbun

Extends DDNS support for the Porkbun v3 JSON API with a custom update
script and service configuration.

See: https://porkbun.com/api/json/v3/documentation

Depends on cURL (with SSL) for transport. Porkbun authentication API keys
and secret keys are passed through the ddns-scripts "username" and
"password" variables, respectively. As Porkbun DNS is currently backed by
Cloudflare, also support ddns-scripts "rec_id" variable for specific
record targeting.

Signed-off-by: Ansel Horn <dev@cahorn.net>