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

Refs #37

internal/cert/dnsproviders/vultr.go

@@ -0,0 +1,20 @@
+package dnsproviders
+
+import (
+	"github.com/caddyserver/certmagic"
+	"github.com/libdns/vultr/v2"
+)
+
+func init() {
+	Register(&Provider{
+		Names:   []string{"vultr"},
+		EnvVars: []string{"VULTR_API_TOKEN"},
+		Build: func(env map[string]string) (certmagic.DNSProvider, error) {
+			token := firstOf(env, "VULTR_API_TOKEN", "VULTR_API_KEY")
+			if token == "" {
+				return nil, missingEnvErr("vultr", "VULTR_API_TOKEN")
+			}
+			return &vultr.Provider{APIToken: token}, nil
+		},
+	})
+}