Add a HNSW collector that exits early when nearest neighbor queue saturates

[tteofili]

This introduces a HnswKnnCollector interface, extending KnnCollector for HNSW, to make it possible to hook into HNSW execution for optimizations.
It then adds a new collector which uses a saturation-based threshold to dynamically halt HNSW graph exploration, in order to early exit when the exploration of new candidates is unlikely to lead to addition of new neighbors.
The new collector records the number of added neighbors upon exploration of a new candidate (a HNSW node) and it compares it with the number of neighbors added while exploring the previous candidate, when the rate of added neighbors plateaus for a number of consecutive iterations, it stops graph exploration (earlyTerminate returns true).

[tteofili]

this sample graph (from Cohere-768) shows how the collection of nearest neighbors saturates and hence it makes sense to stop visiting the graph "earlier", e.g., when the saturation counter exceeds a given threshold.