chore: rename `bucket_count` to `shard_count` in public interface

Summary

• chore: rename bucket_count to shard_count in public interface

Signed-off-by: Anatoly Popov a.popov@picodata.io

---

• Close #1391 (closed)
• Cherry-pick to: none
• Docs follow-up: docs#467 (closed)
• Changelog is updated
• Follow-up for !1653 (merged)