Describe env variables as a part of Parser derive

[pronebird]

Hi,

My binary uses environment variables that aren't bound by clap args but used internally via std::env::var(). Currently I form a custom description with all supported env variables, their respective default values etc. and pass it via after_help.

I wonder if it's possible or worth improvement to provide facilities for defining unbounded env variables via derive macro, for example:

#[derive(Parser)]
#[env(
  name = "MY_ENV_VAR", 
  description = "<description>", 
  default_value_t = <computed_value>,
)]
struct Args {
   // ..
}

Then the new "Environment variables:" section can be appended below the normal "Options" output, i.e:

Usage: my-bin [OPTIONS]

Options:
  -h, --help               Print help
  -V, --version            Print version

Environment variables:
  MY_ENV_VAR               <description> [Default: <default value>]
  .. etc..

[epage]

Personally, this feels a bit of a stretch to include such specific information when we don't interact with it (e.g. providing layered config, see #2763).

However, I have been exploring the idea of a way to attach arbitrary data to arguments / commands along with making the help rendering more extensible which would allow people to implement this on their side.