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/models/server.go

@@ -29,6 +29,7 @@ type Server struct {
 	WgPubKey    string   `json:"wgpubkey,omitempty"`
 	Free        bool     `json:"free,omitempty"`
 	Stream      bool     `json:"stream,omitempty"`
+	Premium     bool     `json:"premium,omitempty"`
 	PortForward bool     `json:"port_forward,omitempty"`
 	Keep        bool     `json:"keep,omitempty"`
 	IPs         []net.IP `json:"ips,omitempty"`