[FLINK-36817] Introduce KafkaConsumerFactory

[lkokhreidze]

In certain scenarios users of the KafkaSource in the flink-connector-kafka might want to provide their own KafkaConsumer. Right now this is not possible as consumer is created in the KafkaPartitionSplitReader which makes customisation impossible.

Proposal is to let users pass KafkaConsumerFactory when building the KafkaSource.

public interface KafkaConsumerFactory {
  KafkaConsumer<byte[], byte[]> get(Properties properties);
}

Builder will have a default implementation which creates the KafkaConsumer the same as it is done now.

[boring-cyborg]

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[lkokhreidze]

Will add the javadocs. But first wanted to get the initial feedback if this makes sense.

[lkokhreidze]

I was debating myself if it would be better to return Consumer interface instead of the implementation. Happy to change it if community agrees that returning an interface is the better choice.

[github-actions]

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