Temporal Sampling - Support "last n" Behavior in C/C++/PLC APIs

[alexbarghi-nv]

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Support selecting the n vertices with the latest timestamp, to allow the temporal_strategy='last' behavior in PyG.

i.e. if fanout is [5, 5, 5] then on each hop for each vertex, we look at all the valid neighbors, order them by timestamp in descending order, and select the latest 5 (or potentially fewer than 5 if sampling without replacement).