MB-66396: Index APIs for TermFrequencies and CentroidCardinalities

[abhinavdangeti]

Requires:

• MB-66396: Add IndexIVF APIs to fetch list of centroids & cardinality faiss#54
• MB-66396: New IndexIVF API: ObtainKCentroidCardinalitiesFromIVFIndex go-faiss#45
• MB-66396: Interface for IndexInsightsReader bleve_index_api#71
• MB-66396: Augment VectorIndex interface scorch_segment_api#65
• MB-66396: New APIs to VectorIndex wrapper zapx#338

[capemox]

Because this function is in snapshot_index_vr.go, the method wouldn't be included in IndexSnapshot if the vectors flag is turned off. So it wouldn't be an implementation of IndexInsightsReader anymore. Calls to TermFrequencies in index_impl.go would then fail because the code checks if the interface is implemented for the reader.

Basically, TermFrequencies would only work if it is compiled with vectors, is this intended?

[abhinavdangeti]

Ya it was intentional - because the insights project is meant for AI applications which will use this alongside vector data. This is just a start though, hoping to add more functions to the interface.

[abhinavdangeti]

Thanks @CascadingRadium for the review so far, will push up a patch reflecting on your comments soon.