Migrate RestTemplateCustomizer to RestClientCustomizer

Migrates Spring Boot's RestTemplateCustomizer to RestClientCustomizer, so that customizers registered as beans keep applying once RestTemplateBuilder is replaced by the auto-configured RestClient.Builder. The RestTemplate parameter of customize methods and lambdas becomes a RestClient.Builder, and the setters that have a counterpart on the builder are renamed (setErrorHandler, setRequestFactory, setMessageConverters, setDefaultUriVariables, setObservationRegistry and setObservationConvention). Other calls on the customized RestTemplate have no builder equivalent and are marked with a TODO comment. Only customizers are changed, so that other RestTemplate usages keep their setters.