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

Refs #37

internal/cert/dnsproviders/registry.go

@@ -0,0 +1,93 @@
+// Package dnsproviders is a registry of DNS providers usable for ACME DNS-01
+// challenges. Each provider is implemented as a separate file that registers
+// itself in init(); to add a new provider, drop a file in this package — no
+// changes are needed elsewhere.
+//
+// All providers wrap the libdns ecosystem (https://github.com/libdns) so they
+// plug directly into certmagic's DNSManager.
+package dnsproviders
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+	"sync"
+
+	"github.com/caddyserver/certmagic"
+)
+
+// Builder constructs a certmagic.DNSProvider from a flat env-style map.
+// The map mirrors the cert_config.dns_env block from config.yml.
+type Builder func(env map[string]string) (certmagic.DNSProvider, error)
+
+// Provider describes a single DNS provider entry in the registry.
+type Provider struct {
+	// Names is the list of accepted dns_provider values; the first is canonical,
+	// the rest are aliases (case-insensitive match).
+	Names []string
+	// EnvVars lists the environment variable names this provider may consume.
+	// Used for documentation and error messages only.
+	EnvVars []string
+	// Build constructs the underlying libdns provider.
+	Build Builder
+}
+
+var (
+	mu       sync.RWMutex
+	registry = map[string]*Provider{}
+	ordered  []*Provider
+)
+
+// Register adds a provider to the global registry. It panics on duplicate
+// names so that registration errors surface at process start.
+func Register(p *Provider) {
+	if p == nil || len(p.Names) == 0 || p.Build == nil {
+		panic("dnsproviders: invalid provider registration")
+	}
+	mu.Lock()
+	defer mu.Unlock()
+	for _, n := range p.Names {
+		key := strings.ToLower(strings.TrimSpace(n))
+		if key == "" {
+			panic("dnsproviders: empty provider name")
+		}
+		if _, exists := registry[key]; exists {
+			panic(fmt.Sprintf("dnsproviders: duplicate name %q", key))
+		}
+		registry[key] = p
+	}
+	ordered = append(ordered, p)
+}
+
+// Get returns the provider registered under the given (case-insensitive) name
+// or alias.
+func Get(name string) (*Provider, bool) {
+	mu.RLock()
+	defer mu.RUnlock()
+	p, ok := registry[strings.ToLower(strings.TrimSpace(name))]
+	return p, ok
+}
+
+// CanonicalNames returns the canonical (first-listed) name of every registered
+// provider, sorted alphabetically. Useful for error messages.
+func CanonicalNames() []string {
+	mu.RLock()
+	defer mu.RUnlock()
+	names := make([]string, 0, len(ordered))
+	for _, p := range ordered {
+		names = append(names, p.Names[0])
+	}
+	sort.Strings(names)
+	return names
+}
+
+// List returns all registered providers (canonical entries only), sorted by
+// canonical name.
+func List() []*Provider {
+	mu.RLock()
+	defer mu.RUnlock()
+	out := make([]*Provider, len(ordered))
+	copy(out, ordered)
+	sort.Slice(out, func(i, j int) bool { return out[i].Names[0] < out[j].Names[0] })
+	return out
+}