springboot Add hypermedia autoconfiguration for RestTemplate.

• Provide RestTemplateCustomizer.
• Verify RestTemplateCustomizer is also picked up for TestRestTemplate.

Related: #16020.

Comment From: gregturn

Per our video chat, we plan to:

• Let Spring HATEOAS provide a programmatic way to register hypermedia with RestTemplate. That way users can leverage it directly. See https://github.com/spring-projects/spring-hateoas/issues/1223 to track that development.
• For Spring Boot 2.3, no autoconfiguration.