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

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[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 90 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.