Auto-configure WebClient.Builder components

[bclozel]

In order to apply #9166 to WebClient instances, we need to customize WebClient.Builder instances with Boot's configuration. One could compare this feature with the RestTemplateBuilder, but there are a few differences:

• WebClient.Builder already exists in webflux
• WebClient.Builder is stateful, meaning we can't expose it as a singleton component, otherwise further customizations made by Spring developers might be applied in different, unintended, places
• WebClient.Builder has a clone method
• WebClient is itself stateless but can expose a new builder instance with its mutate method
• Spring Boot should apply a few auto-configured customizations on the WebClient.Builder (like codecs), and then give that to developers for further changes

[bclozel]

The initial attempt to resolve that is about to be rewritten, once SPR-15690 is resolved.