rename DenseVector/dense_vector field type to KnnVector/knn_vector

[dannynoonan]

Description

Rename DenseVector field type to KnnVector, in the same vein as opensearch-java PR 524

Issues Resolved

Similar to opensearch-java Issue 437

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.31%. Comparing base (ba715b9) to head (9693557).
Report is 70 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #853      +/-   ##
==========================================
- Coverage   71.95%   70.31%   -1.64%     
==========================================
  Files          91      125      +34     
  Lines        8001     9299    +1298     
==========================================
+ Hits         5757     6539     +782     
- Misses       2244     2760     +516     

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[dblock]

Thanks.

1. Sign DCO, git commit --amend -s.
2. Make sure CI passes (nox -rs format to fix linter).
3. Add a test. Let's make sure this code actually works.