Add configuration entry for the reader to define a callback function that collects rebalance events in the form of map[topic name:string][]PartitionAssignment

This information is useful for consumers that need to be aware of what partitions they are assigned to, so that they know when to release resources such as old messages waiting to be processed and committed.

Currently the only way to be aware of these rebalance events is via logs, this PR exposes a cleaner way to collect rebalance events.

Note: runninggo fmt as mentioned in the contributing_guidelines changed comments of functions that I haven't touched, I can revert those changes if needed.