Add mlf-dns-godaddy plugin #89

Uses GoDaddy's Authorization: sso-key KEY:SECRET header. Records are addressed by (domain, type, relative-name); apex records use @ per GoDaddy's convention. PUT /domains/{z}/records/TXT/{name} replaces every record at (type, name) — exactly the single-value shape the _lexicon TXT needs. login does a cheap GET /domains?limit=1 to validate credentials and surface 403s from hobbyist-tier accounts cleanly.