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

Refs #37

internal/cert/dnsproviders/ovh.go

@@ -0,0 +1,29 @@
+package dnsproviders
+
+import (
+	"github.com/caddyserver/certmagic"
+	"github.com/libdns/ovh"
+)
+
+func init() {
+	Register(&Provider{
+		Names: []string{"ovh"},
+		EnvVars: []string{
+			"OVH_ENDPOINT", "OVH_APPLICATION_KEY", "OVH_APPLICATION_SECRET", "OVH_CONSUMER_KEY",
+		},
+		Build: func(env map[string]string) (certmagic.DNSProvider, error) {
+			appKey := firstOf(env, "OVH_APPLICATION_KEY")
+			appSecret := firstOf(env, "OVH_APPLICATION_SECRET")
+			consumerKey := firstOf(env, "OVH_CONSUMER_KEY")
+			if appKey == "" || appSecret == "" || consumerKey == "" {
+				return nil, missingEnvErr("ovh", "OVH_APPLICATION_KEY", "OVH_APPLICATION_SECRET", "OVH_CONSUMER_KEY")
+			}
+			return &ovh.Provider{
+				Endpoint:          firstOf(env, "OVH_ENDPOINT"),
+				ApplicationKey:    appKey,
+				ApplicationSecret: appSecret,
+				ConsumerKey:       consumerKey,
+			}, nil
+		},
+	})
+}