Maint: make VPN connection not specific to OpenVPN

- Add VPN field to ServerSelection struct
- Set VPN type to server selection at start using VPN_TYPE
- Change OpenVPNConnection to Connection with Type field
- Rename Provider GetOpenVPNConnection to GetConnection
- Rename GetTargetIPOpenVPNConnection to GetTargetIPConnection
- Rename PickRandomOpenVPNConnection to PickRandomConnection
- Add 'OpenVPN' prefix to OpenVPN specific methods on connection

internal/configuration/selection.go

@@ -9,6 +9,7 @@ import (
 
 type ServerSelection struct { //nolint:maligned
 	// Common
+	VPN      string `json:"vpn"`
 	TargetIP net.IP `json:"target_ip,omitempty"`
 	// TODO comments
 	// Cyberghost, PIA, Protonvpn, Surfshark, Windscribe, Vyprvpn, NordVPN