Propose a way to be called when an OAuth2 token is refreshed

Currently, the OAuth2AuthInterceptor can refresh the OAuth2 token it has if it comes to expire. This is done under the hood by the interceptor itself, preventing 401 error.

Nevertheless, in some context, we may want to get back the new access and refresh tokens in order to persist them in an external backend for instance.

A possible solution would be to introduce an interface that will acts as a callback called after the tokens are refreshed. The interface should have only classic JDK types.
This mechanism will require new APIs to instantiate the OAuth2AuthInterceptor.