Expose `RabbitBinderConfigurationProperties.enableObservation` property similar to the one in `KafkaBinderConfigurationProperties`

[artembilan]

Right now we do enable observation in RabbitMQ Binder endpoints unconditionally when an ObservationRegistry bean in present in the application context. For consistency it is better to leave the choice up to end-user independently of the environment. See TODO in the RabbitMessageChannelBinder.