Execute KafkaClient reactive and CompletableFuture methods with TaskExecutors.BLOCKING by default

[jeremyg484]

Feature description

An executor configuration option was added in #955 for @KafkaClient beans that enables specifying an ExecutorService to use to make methods that return reactive and CompletableFuture return types fully non-blocking. To avoid breaking existing applications that may already be compensating for the existing behavior with use of facilities such as @ExecuteOn in their calling code, the executor configuration is left empty by default.

We should consider setting this configuration to TaskExecutors.BLOCKING by default in a future major release so that users may take advantage of the reactive style without having to worry about the underlying blocking that happens in calls to the KafkaProducer API. TaskExecutors.BLOCKING is a good fit for this use case since it defaults to virtual threads on Java 21+.

[jeremyg484]

Depending on the timing of the 5.0.0 release vs the release of Kafka 3.8.0, this may not be necessary. Work is in progress in Kafka to refactor the threading model used by KafkaConsumer.

See https://cwiki.apache.org/confluence/display/KAFKA/Consumer+threading+refactor for implementation details and status. If implemented as proposed, the client should be fully non-blocking when using the async APIs.