Add WeakReceiver to tokio::sync::broadcast

[mladedav]

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

#7003 has added WeakSender to tokio::sync::broadcast but there is no WeakReceiver.

We have a use-case where we use a broadcast channel to pass around data to consumers while waiting for Sender::close to resolve so that we kill the processing once there is no one interested in any new data.

We now want to add one special consumer that always exists as long as the Sender exists, but we don't want it to prevent the producing task from exiting.

Describe the solution you'd like

Add tokio::sync::broadcast::WeakReceiver that does not count towards RAII, symetrical to WeakSender.

Describe alternatives you've considered

• To not use Sender::close and instead manually track number of receivers.
• To use multiple channels, one for "Strong" subscribers, one for "Weak" ones.

Additional context

This could also make sense for watch, the other channels only ever have a single receiver.