feat(wireguard): WIREGUARD_IMPLEMENTATION variable

- Can be `auto` (default), `userspace` or `kernelspace`

internal/configuration/sources/env/wireguard.go

@@ -3,6 +3,7 @@ package env
 import (
 	"fmt"
 	"net"
+	"os"
 	"strings"
 
 	"github.com/qdm12/gluetun/internal/configuration/settings"
@@ -15,6 +16,7 @@ func (s *Source) readWireguard() (wireguard settings.Wireguard, err error) {
 	wireguard.PrivateKey = envToStringPtr("WIREGUARD_PRIVATE_KEY")
 	wireguard.PreSharedKey = envToStringPtr("WIREGUARD_PRESHARED_KEY")
 	_, wireguard.Interface = s.getEnvWithRetro("VPN_INTERFACE", "WIREGUARD_INTERFACE")
+	wireguard.Implementation = os.Getenv("WIREGUARD_IMPLEMENTATION")
 	wireguard.Addresses, err = s.readWireguardAddresses()
 	if err != nil {
 		return wireguard, err // already wrapped