Currently, KafkaBinderHealthIndicator is not customizable and included by default
when Spring Boot actuator is on the classpath. Fix this by allowing the application
to provide a custom implementation. A new marker interface called KafkaBinderHealth
can be used by the applicaiton to provide a custom HealthIndicator implementation, in
which case, the binder's default implementation will be excluded.

Tests and docs changes.

Resolves https://github.com/spring-cloud/spring-cloud-stream-binder-kafka/issues/1180