* GH-3635: Add Future<Void> & Mono<Void> to gateway
Fixes https://github.com/spring-projects/spring-integration/issues/3635
When `Future<Void>` & `Mono<Void>` is used as a messaging gateway return type,
the application hangs out on this barrier which may lead to the out of memory eventually
* Add support for the `Future<Void>` & `Mono<Void>` messaging gateway return type
and ensure an asynchronous call for the `gateway.send(Message)` operation and
its exception handling.
In case of successful call, the `Future` is fulfilled with `null` and `Mono` is completed as empty
* * Check for `void.class` as well in the `GatewayProxyFactoryBean.isVoidReturnType`
* * Allow `Future<Void>` as a reply type of the gateway request-reply operation
* * Fix Checkstyle violations
* * Resolve `System.err.println()` in the test code
* Add `Thread.currentThread.interrupt()` to the `InterruptedException` block in the test
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