Add support for Async JsonWebTokenSupplier

[desislava-marinova]

Ditto currently supports access token authentication using only synchronous JsonWebTokenSupplier. Refresh token operation is supposed to be asynchronous one and it will be good if a support for asynchronous JsonWebTokenSupplier is provided, e.g. something like

public interface AsyncJsonWebTokenSupplier extends Supplier<CompletionStage<JsonWebToken>> {
  CompletionStage<JsonWebToken> get();
}