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/constants/openvpn/paths.go

@@ -3,4 +3,7 @@ package openvpn
 const (
 	// AuthConf is the file path to the OpenVPN auth file.
 	AuthConf = "/etc/openvpn/auth.conf"
+	// AskPassPath is the file path to the decryption passphrase for
+	// and encrypted private key, which is pointed by `askpass`.
+	AskPassPath = "/etc/openvpn/askpass" //nolint:gosec
 )