Consumer: Implement graceful closure

[DanielePalaia]

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

The consumer needs to implement a unsettled_message_count in order to allow a graceful shutdown.

Example from Java:

// pause the delivery of messages
consumer.pause();
// ensure the number of unsettled messages reaches 0
long unsettledMessageCount = consumer.unsettledMessageCount();
// close the consumer
consumer.close();

Describe the solution you'd like

Graceful closure to be implemented

Describe alternatives you've considered

No response

Additional context

No response