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

Refs #37

internal/cert/dnsproviders/googleclouddns.go

@@ -0,0 +1,25 @@
+package dnsproviders
+
+import (
+	"github.com/caddyserver/certmagic"
+	"github.com/libdns/googleclouddns"
+)
+
+func init() {
+	Register(&Provider{
+		Names:   []string{"googleclouddns", "gcp", "gcloud"},
+		EnvVars: []string{"GCE_PROJECT", "GCE_SERVICE_ACCOUNT_FILE"},
+		Build: func(env map[string]string) (certmagic.DNSProvider, error) {
+			project := firstOf(env, "GCE_PROJECT", "GOOGLE_PROJECT_ID", "GCP_PROJECT")
+			if project == "" {
+				return nil, missingEnvErr("googleclouddns", "GCE_PROJECT")
+			}
+			// ServiceAccountJSON may be a file path OR a raw JSON blob; when
+			// empty, the default Google credential chain is used.
+			return &googleclouddns.Provider{
+				Project:            project,
+				ServiceAccountJSON: firstOf(env, "GCE_SERVICE_ACCOUNT_FILE", "GOOGLE_APPLICATION_CREDENTIALS"),
+			}, nil
+		},
+	})
+}