Add idempotent publishing code example in languages other than JS

[KacperKluka]

In the messages docs section called Enabling idempotent publishing, the example code that shows how to provide a unique ID for a message is only provided for JavaScript. It would be great to have examples for other languages too. For example, the Java API isn't very clear in terms of this feature and I'm not sure where the unique ID should be provided.

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