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/models/connection.go

@@ -0,0 +1,50 @@
+package models
+
+import (
+	"fmt"
+	"net"
+)
+
+type Connection struct {
+	// Type is the connection type and can be "openvpn"
+	Type string `json:"type"`
+	// IP is the VPN server IP address.
+	IP net.IP `json:"ip"`
+	// Port is the VPN server port.
+	Port uint16 `json:"port"`
+	// Protocol can be "tcp" or "udp".
+	Protocol string `json:"protocol"`
+	// Hostname is used for IPVanish, IVPN, Privado
+	// and Windscribe for TLS verification
+	Hostname string `json:"hostname"`
+}
+
+func (c *Connection) Equal(other Connection) bool {
+	return c.IP.Equal(other.IP) && c.Port == other.Port &&
+		c.Protocol == other.Protocol && c.Hostname == other.Hostname
+}
+
+func (c Connection) OpenVPNRemoteLine() (line string) {
+	return "remote " + c.IP.String() + " " + fmt.Sprint(c.Port)
+}
+
+func (c Connection) OpenVPNProtoLine() (line string) {
+	return "proto " + c.Protocol
+}
+
+// UpdateEmptyWith updates each field of the connection where the
+// value is not set using the value from the other connection.
+func (c *Connection) UpdateEmptyWith(connection Connection) {
+	if c.IP == nil {
+		c.IP = connection.IP
+	}
+	if c.Port == 0 {
+		c.Port = connection.Port
+	}
+	if c.Protocol == "" {
+		c.Protocol = connection.Protocol
+	}
+	if c.Hostname == "" {
+		c.Hostname = connection.Hostname
+	}
+}