Streamline registration of client and subscription beans

Previously

Two approaches were used to register internal beans:

• For subscriptions BeanDefinitionRegistryPostProcessor was used
  • SubscriptionPostProcessor was activated via @Import annotation in the @EnableExternalTaskClient annotation
    • While this works technically, it is not documented, and the behavior might change with future spring releases
• For the client ImportBeanDefinitionRegistrar was used
  • Activating it via @Import annotation is well-documented here

Now

• The client uses ImportBeanDefinitionRegistrar as well
• Both post processors are configured in the newly introduced class PostProcessorConfiguration and imported via @Import annotation in the @EnableExternalTaskClient annotation

Reasoning

Another solution would have been to create a configuration class only for the SubscriptionPostProcessor and import it. However, we used the chance to consolidate how we create internal beans to the BeanDefinitionRegistryPostProcessor approach. The post-processor approach is better documented and more powerful since it makes all beans available in the registry, no matter if these are method-level or class-level exposed beans. Whereas the ImportBeanDefinitionRegistrar only makes class-level beans available in the registry. When @EnableExternalTaskClient on method-level becomes a requirement in the future, we can achieve this easily with the BeanDefinitionRegistryPostProcessor.