Knn imputer implementation

[MichaelFu512]

Pull Request Description

(replace this text with your description)

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After creating the pull request: in order to pass the release_notes_updated check you will need to update the "Future Release" section of docs/source/release_notes.rst to include this pull request by adding :pr:123.

[codecov[bot]]

Codecov Report

Merging #3662 (08dbad6) into main (49090e5) will increase coverage by 0.1%. The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3662     +/-   ##
=======================================
+ Coverage   99.7%   99.7%   +0.1%     
=======================================
  Files        337     339      +2     
  Lines      34078   34239    +161     
=======================================
+ Hits       33947   34108    +161     
  Misses       131     131             

Impacted Files	Coverage Δ
evalml/tests/automl_tests/test_automl.py	99.5% <ø> (ø)
...ts/automl_tests/test_automl_iterative_algorithm.py	100.0% <ø> (ø)
evalml/tests/pipeline_tests/test_pipelines.py	99.8% <ø> (ø)
...lines/components/transformers/imputers/__init__.py	100.0% <100.0%> (ø)
...elines/components/transformers/imputers/imputer.py	100.0% <100.0%> (ø)
...es/components/transformers/imputers/knn_imputer.py	100.0% <100.0%> (ø)
...components/transformers/imputers/simple_imputer.py	100.0% <100.0%> (ø)
evalml/pipelines/components/utils.py	95.8% <100.0%> (+0.5%)	:arrow_up:
evalml/tests/component_tests/test_imputer.py	100.0% <100.0%> (ø)
evalml/tests/component_tests/test_knn_imputer.py	100.0% <100.0%> (ø)
... and 2 more

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