Expose KafkaConsumer in ReceiverOptionsCustomizer.addAssignListener() - parity with KafkaBindingRebalanceListener

Feature Request:

To be able to access the KafkaConsumer when using receiverOptions.addAssignListener:

  @Bean
  public ReceiverOptionsCustomizer customizer() {
    return (bindingName, receiverOptions) -> receiverOptions.addAssignListener(receiverPartitions -> {
      receiverPartitions.forEach(part -> {
      ...

Under the hood this does actually return an instance of SeekablePartition which contains the consumer - unfortunately it isn't publicly accessible.

Why is this useful?

It would provide parity with KafkaBindingRebalanceListener which exposes the hook below.

  @Override
  public void onPartitionsAssigned(String bindingName, Consumer<?, ?> consumer, Collection<TopicPartition> partitions, boolean initial) {

This is useful for microservices that hydrate a cache from Kafka on start-up - as we can ultimately tie consumer.endOffsets(partitions) to a readiness probe once the last offset (per partition) has been consumed.