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 https://github.com/zilliztech/knowhere/pull/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 https://github.com/zilliztech/knowhere/pull/494
• [x] make the parameters configurable

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[github-actions[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions. Rotten issues close after 30d of inactivity. Reopen the issue with /reopen.