springboot Improve Elasticsearch RestClient customization capabilities

At present, RestClientBuilderCustomizer allows general customization of RestClientBuilder. This is troublesome for users that want to customize HttpAsyncClientBuilder and RequestConfig.Builder since those are set on the RestClientBuilder. By customizing those two builders users lose out on Spring Boot's support for binding username, password, connection-timeout and read-timeout properties from spring.elasticsearch.rest namespace.

To illustrate the problem, attempting to customize aspects of HttpAsyncClientBuilder like this:

@Bean
public RestClientBuilderCustomizer restClientBuilderCustomizer() throws Exception {
    SSLContext sslContext; // configure custom sslContext
    SchemeIOSessionStrategy sessionStrategy; // configure custom sessionStrategy
    return builder -> builder.setHttpClientConfigCallback(
            httpClientBuilder -> httpClientBuilder.setSSLStrategy(sessionStrategy).setSSLContext(sslContext));
    });
}

Will disable the binding of username and password properties applied here: https://github.com/spring-projects/spring-boot/blob/70d4994502c848b3db82845c97a033448356c938/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/elasticsearch/ElasticsearchRestClientConfigurations.java#L57-L62

To work around this, users need to replicate Spring Boot's configuration logic using something like:

@Bean
public RestClientBuilderCustomizer restClientBuilderCustomizer(RestClientProperties properties) throws Exception {
    SSLContext sslContext; // configure custom sslContext
    SchemeIOSessionStrategy sessionStrategy; // configure custom sessionStrategy
    return builder -> builder.setHttpClientConfigCallback(httpClientBuilder -> {
        httpClientBuilder.setSSLStrategy(sessionStrategy).setSSLContext(sslContext);
        PropertyMapper.get().from(properties::getUsername).whenHasText().to(username -> {
            CredentialsProvider credentialsProvider = new BasicCredentialsProvider();
            Credentials credentials = new UsernamePasswordCredentials(properties.getUsername(),
                    properties.getPassword());
            credentialsProvider.setCredentials(AuthScope.ANY, credentials);
            httpClientBuilder.setDefaultCredentialsProvider(credentialsProvider);
        });
        return httpClientBuilder;
    });
}

This PR enhances the RestClientBuilderCustomizer with support for customizing HttpAsyncClientBuilder and RequestConfig.Builder by providing additional #customize methods that accept the aforementioned builders. Both new methods are optional as they have no-op default implementations.

Comment From: vpavic

Also, it might be a good idea to expand the javadoc of the original RestClientBuilderCustomizer#customize method (so, the one that takes RestClientBuilder argument) and state that using #setHttpClientConfigCallback and #setRequestConfigCallback on the builder will override the other customizer methods, respectively, as well as the default customizer provided by Spring Boot. WDYT @bclozel?