feat: add VPNsecure.me support (#848)

- `OPENVPN_ENCRYPTED_KEY` environment variable 
- `OPENVPN_ENCRYPTED_KEY_SECRETFILE` environment variable 
- `OPENVPN_KEY_PASSPHRASE` environment variable 
- `OPENVPN_KEY_PASSPHRASE_SECRETFILE` environment variable 
- `PREMIUM_ONLY` environment variable
- OpenVPN user and password not required for vpnsecure provider

internal/provider/vpnsecure/updater/updater.go

@@ -0,0 +1,22 @@
+package updater
+
+import (
+	"net/http"
+
+	"github.com/qdm12/gluetun/internal/provider/common"
+)
+
+type Updater struct {
+	client           *http.Client
+	parallelResolver common.ParallelResolver
+	warner           common.Warner
+}
+
+func New(client *http.Client, warner common.Warner,
+	parallelResolver common.ParallelResolver) *Updater {
+	return &Updater{
+		client:           client,
+		parallelResolver: parallelResolver,
+		warner:           warner,
+	}
+}