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/openvpn/settings.go

@@ -9,11 +9,13 @@ import (
 
 type SettingsGetSetter = state.SettingsGetSetter
 
-func (l *Loop) GetSettings() (settings configuration.OpenVPN) {
+func (l *Loop) GetSettings() (
+	openvpn configuration.OpenVPN, provider configuration.Provider) {
 	return l.state.GetSettings()
 }
 
-func (l *Loop) SetSettings(ctx context.Context, settings configuration.OpenVPN) (
+func (l *Loop) SetSettings(ctx context.Context,
+	openvpn configuration.OpenVPN, provider configuration.Provider) (
 	outcome string) {
-	return l.state.SetSettings(ctx, settings)
+	return l.state.SetSettings(ctx, openvpn, provider)
 }