Function post processing doesn't work with Kotlin classes

[dnijssen]

Describe the bug
We've recently started to implement a PostProcessingFunction (described in https://docs.spring.io/spring-cloud-stream/reference/spring-cloud-stream/post-processing-after-sending-message.html and #1023) however this doesn't seem to work when doing it in Kotlin. The PostProcessionFunction gets wrapped into KotlinFunctionWrapper and therefore it's not recognized as a PostProcessingFunction anymore;

if (target instanceof PostProcessingFunction) {
    this.postProcessor = (PostProcessingFunction) target;
}

Using the following versions:

• Spring Boot :: v3.5.4
• Spring Cloud Stream :: v4.3.0
• Spring Cloud Function :: v4.3.0

So in our case the postProcess function simply doesn't get invoked. As workaround we added a Java equivelent for now, which works. But we actually don't want to be writing Java in our codebases.