Do not reset state when there's only addition of new partitions to the consumer

[mosyp]

Whenever rebalance event is happening, each workers is revoked and then assigned the list of partitions each. As a result, consumer might get new partitions or remove existing. In sake of simplicity and safety in both cases the consumer state is reset and it start consuming messages from previous commit/offset.

However, in theory if the partitions is only added to the consumers and all of existing are kept with a consumer it makes sense to not reset the state for performance reasons.

See https://github.com/delta-io/kafka-delta-ingest/pull/27/files/8e598e35406f5d2978306134ddde9d9d54b4dccc#diff-b1a35a68f14e696205874893c07fd24fdb88882b47c23cc0e0c80a30c7d53759R420 for inspiration