MINOR: Add functionalinterface to the producer callback (apache#19366)

The Callback interface is a perfect example of a place that can use the
functionalinterface in Java. Strictly for Java, this isn't "required"
since Java will automatically coerce, but for Clojure (and other JVM
languages I belive) to interop with Java lambdas it needs the
FunctionalInterface annotation.

Since FunctionalInterface doesn't add any overhead and provides
compiler-enforced documentation, I don't see any reason *not* to have
this. This has already been added into Kafka Streams here:
apache#19234 (review)

I am happy to add it to any other spots in that might be useful too.

Reviewers: Chia-Ping Tsai <[email protected]>

Author: Tombert

clients/src/main/java/org/apache/kafka/clients/producer/Callback.java

@@ -20,6 +20,7 @@
  * A callback interface that the user can implement to allow code to execute when the request is complete. This callback
  * will generally execute in the background I/O thread so it should be fast.
  */
+@FunctionalInterface
 public interface Callback {
 
     /**

connect/runtime/src/main/java/org/apache/kafka/connect/util/Callback.java

@@ -19,6 +19,7 @@
 /**
  * Generic interface for callbacks
  */
+@FunctionalInterface
 public interface Callback<V> {
     /**
      * Invoked upon completion of the operation.