Add FwMark support

[Dim145]

Is your feature request related to a problem? Please describe. I'd like to make sure that client network traffic doesn't use the same interface as wireguard communication. So a different input and output interface.

Describe the solution you'd like Searching on the internet, I found a solution with server wireguard that uses the “FwMark” configuration in the configuration file. Would it be possible to add an option to add this configuration, either in the ui for a value per location or via the gateway environment variables?