Rules do not scan Functional interfaces

[maschnetwork]

Hi,

It seems like the the plugin only detects errors when using the RequestHandler interface. However, when using Spring Cloud Functions you would use the generic java.util.function.Function interface. This will probably also affect other frameworks that are not using the RequestHandler interface.

Examples:

WORKS with RequestHandler (Detects an issue):

@Component
public class LambdaHandler implements RequestHandler<String, String> {

    private final UUID random = UUID.randomUUID();

    @Override
    public String handleRequest(String event, Context context) {
        return random.toString();
    }
}

DOES NOT WORK with Function interface (detects no issues):

@Component
public class LambdaHandler implements Function<String, String> {

    private final UUID random = UUID.randomUUID();

    @Override
    public String apply(String event) {
        return random.toString();
    }
}

Proposal:

I would propose to scan either the entire package or make it configurable.