Allow setting fwmark

[provokateurin]

I'm working on a project using WireGuard and need to be able to set the fwmark for the ping packets. I already do this using code similar to this for other packets:
https://github.com/shadowsocks/shadowsocks-rust/blob/ea6eb27d0865203b69915836739e9e59258ac825/crates/shadowsocks/src/net/sys/unix/linux/mod.rs#L63-L78

I see two options to allow this:

1. Add the ability to pass a custom socket (more flexible, could be useful for other quirks as well)
2. Add the ability to pass the fwmark value and set it inside this crate.

Either one is fine for me and I'm happy to contribute this.
Which one do you prefer?

[aisk]

Hi, thanks for offering to contribute and for starting a discussion before opening a PR!

I would prefer adding a mark option to the builder for setting the socket's fwmark. It should only be available on Linux and Android, just like the existing bind_device option.

[provokateurin]

Thanks for the quick reply! I'll start working on it then.