Introduce new convenience function oauth2_response:to_map/1

This PR takes advantage of the new ```maps:map()``` datatype introduced with Erlang/OTP 17.0 and gives the ability to transform an ```oauth2_response:response()``` to a ```maps:map()```, analog to ```oauth2_response:to_proplist/1```. It is fully compatible with Erlang/OTP releases prior to 17.0.