Add KNN Operator

[metonymic-smokey]

This PR allows a user to specify a KNN Operator to determine if KNN hits are conjuncted/disjuncted with filter query hits.

[metonymic-smokey]

Screenshot of wrong operator with CB Server:

No such issue with blank operator:

Tested with a UT:
searchRequest.AddKNNOperator("bs_op")

Result:

aditiahuja@Aditis-MacBook-Pro bleve % sudo go test --run=TestSimilaritySearchQuery --tags=vectors
Password:
2023/11/20 14:21:38 unknown knn operator: bs_op
exit status 1
FAIL    github.com/blevesearch/bleve/v2 1.362s

[abhinavdangeti]

Looks good, but add some unit tests please.

[abhinavdangeti]

Yo, you've clearly not tested your unit test code -

# github.com/blevesearch/bleve/v2 [github.com/blevesearch/bleve/v2.test]
./search_knn_test.go:92:15: undefined: getQueryVec