[SPARK-3936] Add aggregateMessages, which supersedes mapReduceTriplets

aggregateMessages enables neighborhood computation similarly to mapReduceTriplets, but it introduces two API improvements:

1. Messages are sent using an imperative interface based on EdgeContext rather than by returning an iterator of messages. This is more efficient, providing a 20.2% speedup on PageRank over apache/spark#3054 (uk-2007-05 graph, 10 iterations, 16 r3.2xlarge machines, sped up from 403 s to 322 s).

2. Rather than attempting bytecode inspection, the required triplet fields must be explicitly specified by the user by passing a TripletFields object. This fixes SPARK-3936.

Subsumes apache/spark#2815.
Yeah, we don't currently use it since it's cheap to access the edge attributes, but I think @jegonzal added it in case our internal representation changes and it becomes useful.