xtask: cli flags format

[gabriele-0201]

#221 implementation of the CLI is just a workaround for what I had initially thought. I would prefer to have something like cargo xtask test --zombienet <zombienet_options> --shim <shim_options> instead of multiple options with just the prefix 'zombienet'/'shim'. That is why, in the future, we may add more flags, so having the structs already seems like a good idea. What do you think is the best format for flags here @pepyakin @rphmeier ?

[pepyakin]

I am not entirely sure if/why we need this. However, if we do need this, we could follow the rustc approach.

For example, -Clto=fat or -Zdebug-macros.