sync: add mpsc::Sender::closed future

[hawkw]

Is your feature request related to a problem? Please describe.
Currently, the tokio::sync::watch channel has a Sender::closed async function. This can be awaited in order to notify a task when the receivers of that channel are all closed.

However, the mpsc channel's Sender type does not currently have such a function. This would be useful to add.

For an example use-case, see linkerd/linkerd2#4926. In essence, the intended use case is to prevent leaks of a task that sends messages over an MPSC channel by ensuring it is woken when the receiver ends of that channel are all gone. Currently, the sender can only become aware of the channel closing when it actually attempts to send a message. Although dropping the receivers would wake up the sender task if it is waiting to send a message (in the bounded MPSC), it won't be woken if it is waiting for some other work...but if there are no longer any receivers, that other work may no longer be necessary, and should be cancelled.

If we added a closed future, it would be possible to select over closed and some other work, so that the task is woken when the channel is closed and can proactively cancel itself.

Describe the solution you'd like

We should add a closed future, like the one on watch::Sender.

Describe alternatives you've considered
We could, alternatively, not do this. However, there isn't really any other good way to solve the problem that I can think of.

Additional context
Somewhat related: #2481

[taiki-e]

Closing -- done in #2840