Consider deprecating RestClientBuilderConfigurer

[philwebb]

WebClientAutoConfiguration provides a WebClient.Builder prototype bean for use where as RestClientAutoConfiguration offers a RestClientBuilderConfigurer. I'm wondering if we really need the configurer and how common it is to want to configure an existing builder.

[philwebb]

We discussed this today and decided not to remove anything in 4.0. We will consider deprecating the class at a later date.