Document external version with custom modes supported

[hongphu8790]

Here are issues with fixed ES document version: https://github.com/confluentinc/kafka-connect-elasticsearch/issues/142 https://github.com/confluentinc/kafka-connect-elasticsearch/issues/308

I implemented the feature for user need customize the ES external version by kafka message offset, kafka message timestamp, or default with legacy operation.

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[hongphu8790]

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[klinika]

Hello, i have same problem with no custom algorithm for ES version. Your decision really help me. Can you add documentVersionType HEADER with param HEADER_KEY for reading version from kafka topic header? With best regards, Dmitiry.

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[YeonghyeonKO]

Is there any update after this PR?