Enable custom binder health check impelementation

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

docs/src/main/asciidoc/overview.adoc

@@ -971,3 +971,28 @@ public AdminClientConfigCustomizer adminClientConfigCustomizer() {
     };
 }
 ```
+
+[[custom-kafka-binder-health-indicator]]
+=== Custom Kafka Binder Health Indicator
+
+Kafka binder activates a default health indicator when Spring Boot actuator is on the classpath.
+This health indicator checks the health of the binder and any communication issues with the Kafka broker.
+If an application wants to disable this default health check implementation and include a custom implementation, then it can provide an implementation for `KafkaBinderHealth` interface.
+`KafkaBinderHealth` is a marker interface that extends from `HealthIndicator`.
+In the custom implementation, it must provide an implementation for the `health()` method.
+The custom implementation must be present in the application configuration as a bean.
+When the binder discovers the custom implementation, it will use that instead of the default implementation.
+Here is an example of such a custom implementation bean in the application.
+
+```
+@Bean
+public KafkaBinderHealth kafkaBinderHealthIndicator() {
+    return new KafkaBinderHealth() {
+        @Override
+        public Health health() {
+            // custom implementation details.
+        }
+    };
+}
+```
+