Add TaskLocalFuture::into_value

[TennyZhuang]

Is your feature request related to a problem? Please describe.

Sometimes we want to use LocalKey with Stream, and it's impossible to yield from an async closure. A workaround is wrap inside the stream iteration:

tokio::task_local! {
    static TASK_LOCAL_ALLOCATED_BYTES: AtomicUsize;
}

let allocated_bytes = AtomicUsize::new(0);
while let Some(message) =
        TASK_LOCAL_ALLOCATED_BYTES.scope(allocated_bytes, input.try_next()).await?
    {
        yield message;
    }

However, the scope will take the ownership of the slot value, and compile failed.

Describe the solution you'd like

I hope we can add a method to return the ownership of slot value instead of always dropping it, so that we can:

    let mut allocated_bytes = Some(AtomicUsize::new(0));

    loop {
        let f = TASK_LOCAL_ALLOCATED_BYTES.scope(allocated_bytes.take().unwrap(), input.try_next());

        let message = (&mut f).await?;
        if let Some(message) = message {
            yield message;
            allocated_bytes = Some(f.into_value());
        }
    }

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Directly support Stream, such as LocalKey::scope_stream.

Additional context
Add any other context or screenshots about the feature request here.

[Darksonn]

This seems reasonable to me.

[Darksonn]

As pointed out in #5758 (comment), this method would not be callable once the future has been pinned. A take_value method might be a better idea?