improve the iterator implementation

[zhengbuqian]

so that we can avoid as much duplicate code between indexes as possible and form a good foundation for Cardinal to add iterator support

[zhengbuqian]

TODOs after #501:

• hnsw iterator use brute force for high filter rate
• hnsw iterator to limit to_visit size by search 1 step every several Next() calls.
• ivf iterator to copy and own query data
• DiskANN iterator impl
• IVF-SQ8 iterator impl: this is being targeted in Add iterator for ivf_sq_8 #414
• HNSW impl has support for binary vector, but it is not officially supported. Thus hnsw iterator on binary vector is not properly tested: this is being targeted in hnsw support fp16/bf16 #494
• make the parameters configurable

[github-actions]

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