springboot Add hypermedia autoconfiguration for WebClient.

• Provide CodecCustomizer to support WebClient.

Related: #16020

Comment From: gregturn

@wilkinsona I applied the tweaks mentioned in #20372 to this PR plus a bunch of polishing to get it green on Concourse.

If we can finish this one out, I can rebase #20372 to use it.

Comment From: gregturn

Per our video chat,

• Provide API in Spring HATEOAS to ease configuring WebClient instances. See https://github.com/spring-projects/spring-hateoas/issues/1224 to track that development.
• No autoconfiguration for WebClient in Spring Boot 2.3.