Maint: refactor VPN configuration structure

- Paves the way for Wireguard
- VPN struct contains Type, Openvpn and Provider configurations
- OpenVPN specific options (e.g. client key) moved from Provider to Openvpn configuration struct
- Move Provider configuration from OpenVPN configuration to VPN
- HTTP control server returns only openvpn settings (not provider settings)

internal/constants/vpn.go

@@ -1,5 +1,9 @@
 package constants
 
+const (
+	OpenVPN = "openvpn"
+)
+
 const (
 	// Cyberghost is a VPN provider.
 	Cyberghost = "cyberghost"