feat(cert): add registry-based DNS provider system with 21 libdns providers

Refs #37

internal/cert/dnsproviders/godaddy.go

@@ -0,0 +1,30 @@
+package dnsproviders
+
+import (
+	"github.com/caddyserver/certmagic"
+	"github.com/libdns/godaddy"
+)
+
+func init() {
+	Register(&Provider{
+		Names:   []string{"godaddy"},
+		EnvVars: []string{"GODADDY_API_TOKEN"},
+		Build: func(env map[string]string) (certmagic.DNSProvider, error) {
+			// GoDaddy auth header format: "sso-key KEY:SECRET"
+			// Pass the full "KEY:SECRET" pair via GODADDY_API_TOKEN, OR set
+			// GODADDY_API_KEY + GODADDY_API_SECRET and we'll join them.
+			token := firstOf(env, "GODADDY_API_TOKEN")
+			if token == "" {
+				key := firstOf(env, "GODADDY_API_KEY")
+				secret := firstOf(env, "GODADDY_API_SECRET")
+				if key != "" && secret != "" {
+					token = key + ":" + secret
+				}
+			}
+			if token == "" {
+				return nil, missingEnvErr("godaddy", "GODADDY_API_TOKEN", "GODADDY_API_KEY+GODADDY_API_SECRET")
+			}
+			return &godaddy.Provider{APIToken: token}, nil
+		},
+	})
+}