add dataAccessExpirationTime property to ClassicToken

[idish]

Hi, Please do not merge this until fixing the code setting the value of dataAccessExpirationTime in facebook_auth_desktop. I tested the code only on mobile and couldn't properly test on desktop, so please do and change dataAccessExpirationTime: DateTime.fromMicrosecondsSinceEpoch(0) to set the value properly.

dataAccessExpirationTime property is a major property that became unavailable after version 7.0.0. As this property encapsulates the time we can use our access token for data access, and not just for authentication (the current expires property, which effectively never expires anyway).