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/resolve.go

@@ -0,0 +1,26 @@
+package updater
+
+import (
+	"time"
+
+	"github.com/qdm12/gluetun/internal/updater/resolver"
+)
+
+func parallelResolverSettings(hosts []string) (settings resolver.ParallelSettings) {
+	const (
+		maxDuration  = 5 * time.Second
+		maxFailRatio = 0.1
+		maxNoNew     = 2
+		maxFails     = 3
+	)
+	return resolver.ParallelSettings{
+		Hosts:        hosts,
+		MaxFailRatio: maxFailRatio,
+		Repeat: resolver.RepeatSettings{
+			MaxDuration: maxDuration,
+			MaxNoNew:    maxNoNew,
+			MaxFails:    maxFails,
+			SortIPs:     true,
+		},
+	}
+}