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/firewall/firewall.go

@@ -39,7 +39,7 @@ type Config struct { //nolint:maligned
 
 	// State
 	enabled           bool
-	vpnConnection     models.OpenVPNConnection
+	vpnConnection     models.Connection
 	outboundSubnets   []net.IPNet
 	allowedInputPorts map[uint16]string // port to interface mapping
 	stateMutex        sync.Mutex

internal/firewall/iptables.go

@@ -150,7 +150,7 @@ func (c *Config) acceptEstablishedRelatedTraffic(ctx context.Context, remove boo
 }
 
 func (c *Config) acceptOutputTrafficToVPN(ctx context.Context,
-	defaultInterface string, connection models.OpenVPNConnection, remove bool) error {
+	defaultInterface string, connection models.Connection, remove bool) error {
 	instruction := fmt.Sprintf("%s OUTPUT -d %s -o %s -p %s -m %s --dport %d -j ACCEPT",
 		appendOrDelete(remove), connection.IP, defaultInterface, connection.Protocol,
 		connection.Protocol, connection.Port)

internal/firewall/vpn.go

@@ -8,10 +8,10 @@ import (
 )
 
 type VPNConnectionSetter interface {
-	SetVPNConnection(ctx context.Context, connection models.OpenVPNConnection) error
+	SetVPNConnection(ctx context.Context, connection models.Connection) error
 }
 
-func (c *Config) SetVPNConnection(ctx context.Context, connection models.OpenVPNConnection) (err error) {
+func (c *Config) SetVPNConnection(ctx context.Context, connection models.Connection) (err error) {
 	c.stateMutex.Lock()
 	defer c.stateMutex.Unlock()
 
@@ -33,7 +33,7 @@ func (c *Config) SetVPNConnection(ctx context.Context, connection models.OpenVPN
 			c.logger.Error("cannot remove outdated VPN connection through firewall: " + err.Error())
 		}
 	}
-	c.vpnConnection = models.OpenVPNConnection{}
+	c.vpnConnection = models.Connection{}
 	remove = false
 	if err := c.acceptOutputTrafficToVPN(ctx, c.defaultInterface, connection, remove); err != nil {
 		return fmt.Errorf("cannot set VPN connection through firewall: %w", err)