Support `Option<T>` when flattening

[epage]

Please complete the following tasks

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

Clap Version

4.0.0

Describe your use case

When flattening, I'd like to make the struct optional

#[derive(Clap)]
pub struct App{
    #[clap(flatten)]
    pub state_filter: Option<StateFilter>,
}

#[derive(Args)]
pub enum StateFilter {
    /// Only running servers are returned
    #[clap(long)]
    Running,
    /// Only exited servers are returned
    #[clap(long)]
    Exited,
    /// Only restarting servers are returned
    #[clap(long)]
    Restarting,
    #[clap(long)]
    Custom(String),
}

Describe the solution you'd like

With #3165, we could check if the ArgGroup is present and, if it isn't, return None. Otherwise, we walk into the ArgGroup and access it.

Alternatives, if applicable

No response

Additional Context

No response

[epage]

Oops, looks like we already have #3123, closing in favor of that though #4350 is just about to be merged, supporting this

[epage]

v4.0.10