Improve client configuration callbacks

[sothawo]

Client configuration:

        @Bean
	public ClientConfiguration clientConfiguration() {
		return ClientConfiguration.builder() //
			.connectedTo("localhost:9200") //
			.withClientConfigurer(ReactiveRestClients.WebClientConfigurationCallback.from(webClient -> {
				// will be called when a reactive client is created
				return webClient;
			}))
			.withClientConfigurer(ReactiveRestClients.WebClientConfigurationCallback.from(webClient -> {
				// will also be called when a reactive client is created
				return webClient;
			}))
			.withClientConfigurer(RestClients.RestClientConfigurationCallback.from(clientBuilder -> {
				// will be called when an imperative client is created
				return clientBuilder;
			}))
			.build();
	}

Closes #1929