feat(wireguard): WIREGUARD_IMPLEMENTATION variable

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

internal/provider/utils/wireguard.go

@@ -14,6 +14,7 @@ func BuildWireguardSettings(connection models.Connection,
 	settings.PublicKey = connection.PubKey
 	settings.PreSharedKey = *userSettings.PreSharedKey
 	settings.InterfaceName = userSettings.Interface
+	settings.Implementation = userSettings.Implementation
 	settings.IPv6 = &ipv6Supported
 
 	const rulePriority = 101 // 100 is to receive external connections