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/provider/windscribe/openvpnconf.go

@@ -67,7 +67,7 @@ func (w *Windscribe) BuildConf(connection models.OpenVPNConnection,
 		lines = append(lines, "mssfix "+strconv.Itoa(int(settings.MSSFix)))
 	}
 
-	if settings.Provider.ExtraConfigOptions.OpenVPNIPv6 {
+	if settings.IPv6 {
 		lines = append(lines, "tun-ipv6")
 	} else {
 		lines = append(lines, `pull-filter ignore "route-ipv6"`)