Add support for RestClient #941
Description
To ease transition from RestOperations (respective RestTemplate), we should add RestClient support and use RestClient internally in authentication and VaultTemplate.
Given the already existing doWithSession and doWithVault methods, we would create an unpleasant clash by introducing overloads accepting a RestClientCallback callback argument for lambda implementations as existing code would now have to declare which overload to use by explicitly selecting RestOperationsCallback. For the time being it seems better to have these methods as package-private variants as opportunity for further evolution towards a VaultClient architecture that exposes low-level operations applicable for the entire Vault API to decouple the immediate coupling between the Template API and RestClient/Operations.
Having a RestClientBuilder and accepting RestClient at VaultTemplate would make a decent initial support arrangement.