Annotate Java facade listener interfaces as functional

[QuintinWillison]

See https://www.baeldung.com/java-8-lambda-expressions-tips#functionalInterface for reasons behind this being a good idea / best practice.

[KacperKluka]

@QuintinWillison I've checked and the @FunctionalInterface annotation only works in .java files. If it's used in a .kt file it has no effect at all and you can declare multiple methods in the annotated interface without any errors.
So if we'd really like to use that annotation then we would have to refactor those interfaces from Kotlin to Java. WDYT?

[QuintinWillison]

Thanks for checking. That is interesting. Refactoring these interfaces to Java is probably a very good idea. I can't imagine there are any Kotlin features that the facade projects (for publishing and subscribing) are using that would stop us doing this, but that would clearly fall out when it comes to carrying out this task.

[QuintinWillison]

Relates to #174, in that we could present the Java facade interfaces as a Java project and then implement them in Kotlin via a second project.

i.e., ideally, we would keep the existing Kotlin projects subscribing-sdk-java and publishing-sdk-java as facade implementations, and add new Java projects subscribing-sdk-java-api and publishing-sdk-java-api which would be pure interface declarations, leveraging @FunctionalInterface annotation where appropriate.