AdminClient customizer (#1023)

* AdminClient customizer

Provide the ability for applications to customize AdminClient by
introducing a new interface AdminClientConfigCustomizer.

Resolves https://github.com/spring-cloud/spring-cloud-stream-binder-kafka/issues/1014

* Addressing PR review comments

docs/src/main/asciidoc/overview.adoc

@@ -786,3 +786,20 @@ For example, if you want to gain access to a bean that is defined at the applica
 When the binder discovers that these customizers are available as beans, it will invoke the `configure` method right before creating the consumer and producer factories.
 
 Both of these interfaces also provide access to both the binding and destination names so that they can be accessed while customizing producer and consumer properties.
+
+[[admin-client-config-customization]]
+=== Customizing AdminClient Configuration
+
+As with consumer and producer config customization above, applications can also customize the configuration for admin clients by providing an `AdminClientConfigCustomizer`.
+AdminClientConfigCustomizer's configure method provides access to the admin client properties, using which you can define further customization.
+Binder's Kafka topic provisioner gives the highest precedence for the properties given through this customizer.
+Here is an example of providing this customizer bean.
+
+```
+@Bean
+public AdminClientConfigCustomizer adminClientConfigCustomizer() {
+    return props -> {
+        props.put(CommonClientConfigs.SECURITY_PROTOCOL_CONFIG, "SASL_SSL");
+    };
+}
+```