KIP-706 — Add method "Producer#send" to return CompletionStage instead of Future

Discussion Producer

Adds a `sendAsync()` method to the `Producer` interface that returns `CompletionStage<RecordMetadata>` instead of `Future<RecordMetadata>`, and deprecates the existing `send()` methods. `Future`'s limited API makes it difficult to write non-blocking asynchronous producer code; `CompletionStage` (available since Java 8) supports chaining, composition, and integration with async frameworks.

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