Uniform AsyncRuntime channel interfaces

[SteveLauC]

Trait AsyncRuntime exposes various associated channel types:

• Oneshot
• MPSC
• Watch

Currently, both MPSC and Watch are exposed through traits:

https://github.com/datafuselabs/openraft/blob/ec284cb4813019d8b3a43a8cd74f3e1aa39eb882/openraft/src/type_config/async_runtime/mod.rs#L120-L122

Oneshot is different, it is scattered in the trait:

pub trait AsyncRuntime: Debug + Default + PartialEq + Eq + OptionalSend + OptionalSync + 'static {
    // Other fields are omitted

    type OneshotSender<T: OptionalSend>: OneshotSender<T> + OptionalSend + OptionalSync + Sized;
    type OneshotReceiverError: std::error::Error + OptionalSend;
    type OneshotReceiver<T: OptionalSend>: OptionalSend
        + OptionalSync
        + Future<Output = Result<T, Self::OneshotReceiverError>>
        + Unpin;
    fn oneshot<T>() -> (Self::OneshotSender<T>, Self::OneshotReceiver<T>)
    where T: OptionalSend;
}

To provide a consistent interface, I kinda think we should expose Oneshot in the way used by MPSC and Watch, i.e., something like:

pub trait Oneshot {
    type Sender<T: OptionalSend>: OneshotSender<T>;
    type Receiver<T: OptionalSend>: OptionalSend
        + OptionalSync
        + Future<Output = Result<T, Self::ReceiverError>>
        + Unpin;
    type ReceiverError: std::error::Error + OptionalSend;

    fn channel<T>() -> (Self::Sender<T>, Self::Receiver<T>)
    where T: OptionalSend;
}

pub trait AsyncRuntime: Debug + Default + PartialEq + Eq + OptionalSend + OptionalSync + 'static {
    // Other fields are omitted

    type MpscUnbounded: MpscUnbounded; 
    type Watch: Watch; 
    type Oneshot: Oneshot;
}

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[SteveLauC]

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