add toml support, like env support

Please complete the following tasks

[X] I have searched the discussions
[X] I have searched the open and rejected issues

Clap Version

4.5.8

Describe your use case

creating a cli tool to run a server which could allow configuration via environment variables or toml

Describe the solution you'd like

a toml features which allows a toml key to be specified in the arg macro:

#[derive(Parser)]
pub struct Cli {
    #[arg(env = "FOO", toml = "foo")]
    foo: String
}

I think by default the priority should be passed argument, env, toml, default. though this should be configureable

Alternatives, if applicable

the main alternative is to manually parse a toml file, the problem with this that that it is difficult to differentiate env or passed values from clap and the default

Additional Context

If this is a feature that there is support for I'd love to implement it

clap-rs / clap