Allow configurable RestOperations for DefaultOAuth2UserService #5600
Description
DefaultOAuth2UserService uses Nimbus's HTTP Client internally to retrieve the UserInfo resource.
We should re-factor to use RestOperations instead and additionally expose a DefaultOAuth2UserService.setRestOperations(restOperations) to allow user's the ability to supply a pre-configured RestOperations that are capable of adding custom parameters and/or headers.
Related #5601