Added new IConsumer.Consume overload taking target ConsumeResult/Message as param for low-alloc flows.

[UBSaxo]

This PR adds a new IConsumer.Consume(int, ConsumeResult) overload that allows caller to bring his own (likely reused) ConsumeResult with embedded Message.

This makes it easy for caller to reuse single ConsumeResult instance and single Message instance and thereby reduce overall allocations.

Headers class has also been optimized internally to only create the backing list when first header is added. This avoids allocating an empty list for all messages without headers.

[confluent-cla-assistant]

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✅ UBSaxo
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