Add support for requesting protected resources with RestClient

[mjeffrey]

Expected Behavior Allow the use RestClient (to be introduced in Spring 6.1) for blocking calls in a non reactive application In Oauth2 Client. See https://spring.io/blog/2023/07/13/new-in-spring-6-1-restclient.

Current Behavior Only WebClient is supported which means a lot of reactive dependencies are pulled in when using Oauth2 Client even in a blocking application.

Context To avoid the dependency issue we are using RestTemplate (with interceptors) but would prefer to see a supported solution.