Support parsing smart pointers

[KSXGitHub]

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Clap Version

4.3.8

Describe your use case

Box of an unsized type is often smaller than its owned counterpart. And it is cheaper to clone Rc and Arc than cloning the owned version.

Describe the solution you'd like

Allow parsing arguments directly to smart pointer types such as Box, Rc, and Arc. For example:

#[derive(Clone, Parser)]
struct Args {
    #[clap(long, short)]
    tag: Arc<[Box<str>]>, // instead of Vec<String>
    files: Arc<[Box<Path>]>, // instead of Vec<PathBuf>
}

Alternatives, if applicable

No response

Additional Context

No response

[epage]

#4626 is a more general re-evaluation of how we infer meaning from types