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/vpn/interfaces.go

@@ -29,6 +29,7 @@ type PortForward interface {
 type OpenVPN interface {
 	WriteConfig(lines []string) error
 	WriteAuthFile(user, password string) error
+	WriteAskPassFile(passphrase string) error
 }
 
 type Providers interface {