Add support for Component classes for spring cloud stream plugin (#684)

[stavshamir]

Until now, cloud stream plugin could auto detect Consumer/Supplier/Function bean methods:

@Configuration
public class MyConsumerConfiguration {
  @Bean
  public Consumer<String> myConsumer() {
    return log::info;
  }
}

But the following valid configuration was not detected:

@Component
public class MyConsumer implements Consumer<String> {
  @Override
  public void accept(String payload) {
    log.info(payload);
  }
}

This PR adds support for auto-detections of Component class in cloud stream plugin. These now lead to the creation of channels and operations automatically.

Additionally, to support this feature ChannelBindingProcessor now accepts AnnotatedElement instead of Method. Bindings could now potentially be used on classes as well, but I did not change the only existing implementation (google pubub) for now.

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