Support include message key to metadata

[shibd]

Motivation

Support include message key to metadata

Modifications

• Support include message key to metadata

Verifying this change

• testExtractedMetadata will cover this change.
• Integration test, send to S3 data like:

{
	"name": "test22",
	"age": 96,
	"email": "ass@gmail.com",
	"__message_metadata__": {
		"publishTime": 1752590253531,
		"messageKey": "key96",
		"schemaVersion": "AAAAAAAAAAA=",
		"messageId": "CKoJEGIYADAA",
		"topic": "persistent://public/default/s3-sink-partition-0",
		"properties": {}
	}
}

Documentation

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• no-need-doc

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• doc

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[github-actions]

@shibd:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)