Derive feature: Add a prefix for env vars

[remoun]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Clap Version

3.1.2

Describe your use case

I'm working on adding env var support to a fairly large number of flags, with the Derive API. It would be great if we could specify a common prefix for all the env vars, without having to spell it out for each flag.

Describe the solution you'd like

Something like all_env_prefix = "FOO_", e.g.

#[derive(Debug, Parser)]
#[clap(rename_all_env = "kebab", all_env_prefix="FOO_")]
struct BehaviorModel {
    #[clap(env)]
    be_nice: String,
}

Alternatives, if applicable

The current alternative is to spell out each of the names with the env = "..." syntax, but that's very repetitive.

A more general approach would be to allow specifying a callback for mapping flag names to env vars, e.g.

#[derive(Debug, Parser)]
#[clap(rename_all_env = flag_to_env)]
struct BehaviorModel {
    #[clap(env)]
    be_nice: String,
}

pub fn flag_to_env(name: &str): String {
    use heck::ToShoutySnakeCase;
    return format!("FOO_{}", name.to_shouty_snake_case());
}

... but I'm not sure that's worth the overhead of supporting callbacks, for which there is no precedent AFAIK.

Additional Context

This seems similar to #2463 but for env vars renaming.

[remoun]

Oops, I guess this is a dupe of #3221