Support consumer event listener

[dfarr]

Describe the solution you'd like
It would be awesome if the go client supported the consumer event listener, similar to the java client.

Is your feature request related to a problem? Please describe.
We have an application that uses a partitioned topic. On application scale out, we need to know which consumers have been unassigned from a topic/partition in order to clear the internal memory of our application for this partition.

Describe alternatives you've considered
We have explored a Key_Shared subscription type with a non-partitioned topic. When a scale out of the application occurs with this type of subscription unacked messages are not automatically re-delivered and message consumption is paused until the pending messages are acked/nacked. See apache/pulsar#15705. This behaviour prevents our application from using a Key_Shared subscription type.

Additional context

• slack thread
• related stackoverflow question

[labuladong]

Assign to me @codelipenghui