Allow argument-less `default_missing_value` attribute

[flying-sheep]

Please complete the following tasks

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

Clap Version

4.5.54

Describe your use case

I don’t like that I have to repeat myself here:

#[derive(Debug, Parser)]
struct Args {
    #[arg(long, num_args = ..=1, require_equals = true, default_missing_value = "the-default")]
    some_opt: Option<ThingImplementingDefault>,
}

#[derive(Debug, Default, Clone, clap::ValueEnum)]
enum ThingImplementingDefault {
    #[default]
    TheDefault,
    SomethingElse,
}

Describe the solution you'd like

Would be great if Optional<ThingImplementingDefault> could be made to behave like this without repetition:

• nothing -> None
• --some-opt -> Some(Default::default())
• --some-opt=something-else -> Some(value.parse()?)

Maybe just default_missing_value without argument would behave like this?

Alternatives, if applicable

Implement default_missing_value_t, then I could do default_missing_value_t = Some(Default::default())

Additional Context

See also #6109 for more default_missing_value utility

[epage]

I think this would be reasonable for default_missing_value_t though that needs to be implemented first, see #2813.