Supports aggregation of past user interactions as additional user/context features

[benfred]

NVTabular should create ops it easier the process of aggregating some past user interactions to represent users preferences. Some possible approaches would be:

• [ ] Including features of the last item interacted by the user as additional user/context features (like done in this Google paper on retrieval models, where the provide the last seen video ("seed video") as context features)
• [ ] Aggregating item features from past user interactions (i.e. excluding the current and future interaction). For example, being able to aggregate by user some item features like preferred categories, genres, avg. products prices (but only for past interactions).