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/configuration/selection.go

@@ -33,7 +33,7 @@ type ServerSelection struct { //nolint:maligned
 	// NordVPN
 	Numbers []uint16 `json:"numbers"`
 
-	// PIA
+	// PIA - needed to get the port number
 	EncryptionPreset string `json:"encryption_preset"`
 
 	// ProtonVPN
@@ -43,13 +43,6 @@ type ServerSelection struct { //nolint:maligned
 	StreamOnly bool `json:"stream_only"`
 }
 
-type ExtraConfigOptions struct {
-	ClientCertificate string `json:"-"`                 // Cyberghost
-	ClientKey         string `json:"-"`                 // Cyberghost, VPNUnlimited
-	EncryptionPreset  string `json:"encryption_preset"` // PIA
-	OpenVPNIPv6       bool   `json:"openvpn_ipv6"`      // Mullvad
-}
-
 // PortForwarding contains settings for port forwarding.
 type PortForwarding struct {
 	Enabled  bool   `json:"enabled"`