SpringBoot WebClient auto-configuration tries to use HttpComponentsClientHttpConnector when all required classes are not present

As reported in https://github.com/spring-projects/spring-boot/issues/34962, a java.lang.NoClassDefFoundError: org/apache/hc/core5/reactive/ReactiveResponseConsumer can occur when an incomplete set of Http Client 5 modules is on the classpath. We need to tighten up the classpath checks to ensure that ReactiveResponseConsumer is on the classpath before auto-configuring HttpComponentsClientHttpConnector.