Add builder constructor that takes algo params

[jmazanec15]

Issue #, if available:
#288

Description of changes:
In this PR, I added a constructor for KNNVectorFieldMapper.Builder that takes the algorithm parameters that it typically reads from settings. This constructor is used during merging of mappers. It allows an existing KNNVectorFieldMapper to pass settings to a KNNVectorFieldMapper.Builder for merge. It plays a similar role to the ParametrizedFieldMapper.init method, which copies mapping parameters from an already existing FieldMapper to the FieldMapper.Builder that will be used for merging. I decided to not override this method because it is more focused on mapping parameters, but I am open to feedback on this.

Verified that with this change, I am unable to reproduce the issue following the steps in #288.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Merging #289 (72f247d) into master (c41f375) will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #289      +/-   ##
============================================
+ Coverage     80.38%   80.51%   +0.13%     
  Complexity      388      388              
============================================
  Files            62       62              
  Lines          1458     1468      +10     
  Branches        127      130       +3     
============================================
+ Hits           1172     1182      +10     
  Misses          239      239              
  Partials         47       47              

Impacted Files	Coverage Δ	Complexity Δ
...relasticsearch/knn/index/KNNVectorFieldMapper.java	80.68% <100.00%> (+1.43%)	16.00 <1.00> (ø)

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[vamshin]

Good catch. LGTM!

[VijayanB]

Is it possible to add integration test or unit test for this scenario? May be as another commit since this is critical.

[jmazanec15]

Is it possible to add integration test or unit test for this scenario? May be as another commit since this is critical.

Yes, I think we need to add more end to end testing in our integration tests. We can take that as another task.