feat: add initial support for split tunnel

[fcastilloec]

This address / fixes #62

It might not be perfect, it could use some extra checks (i.e. a valid IP address for virtual interfaces), or better names for the scripts. Nonetheless, it's a start for other people to use them.

I've tested them with Wireguard and OpenVPN, and they are working without any issue

[g00nix]

I think this is a nice addition and could develop into scripts that everybody could use, however there are still some things to clear out:

• the scripts should allow you to isolate any application, not just Firefox
• maybe this should be in a separate folder as most people will never use this
• this might be better to stay in a fork, as it goes a bit out of the scope of this repo

I will discuss this internally to see if we can expand the scope of this repo in 2021, but up to that point I think a fork is better for specific network configuration that only a few users might use.

[fcastilloec]

• the scripts should allow you to isolate any application, not just Firefox

Firefox is just an example as stated in the message, any program can be started and will work. In fact, it's required of the user to run ip netns exec $NETNS_NAME ./run_setup.sh in order to start the VPN. I can make it so that we call run_setup.sh inside the script, rather than asking the user to do so.

The command to start any app is: ip netns exec $NETNS_NAME [path_to_any_executable]
You could even just start bash, ip netns exec $NETNS_NAME bash, and run any other command which will use the VPN connection (as long as the VPN has started with the previous command).

[jahutchi]

I've been using these also in order to launch specific applications in the namespace.

One thing I found in the create_namespace.sh is that I had to add the following:
echo 1 > /proc/sys/net/ipv4/ip_forward

[aarsenca]

Is this the proper place to ask for help with create_namespace.sh? Thanks.

[fcastilloec]

@aarsenca I no longer use these scripts but I could answer some of your questions.
Even though I don't think this PR will get merged, it might be better if you message me at https://twitter.com/fcastillo