add parser functionality to RethCli

[mattsse]

with #9082 we now have a rethcli type, we need to integrate args parsing by integrating clap's parser in it.

this can be done by adding functions like

fn parse_args() -> Self where Self: Parser + Sized

https://docs.rs/clap/latest/clap/trait.Parser.html

[leruaa]

Not sure if I fully understood what you want, but I can give a try :)

[mattsse]

for now we basically should just delegate to the clap::Parser's functions for parse from env args

we also need fn try_parse_from<I, T>(itr: I), then the parse fn simply calls this with the std::env::args_os()