Change "space" parameter to "space_type" for custom scoring

[jmazanec15]

Issue #, if available:

Description of changes:
For Approximate k-NN search, we use "space_type" to specify the space for the index. For custom scoring search, we use the parameter "space". To maintain consistency and prevent confusion, this PR changes the parameter "space" to "space_type" for custom scoring.

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[codecov]

Codecov Report

Merging #232 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #232   +/-   ##
=========================================
  Coverage     78.51%   78.51%           
  Complexity      335      335           
=========================================
  Files            53       53           
  Lines          1308     1308           
  Branches        121      121           
=========================================
  Hits           1027     1027           
  Misses          228      228           
  Partials         53       53           

Impacted Files	Coverage Δ	Complexity Δ
...search/knn/plugin/script/KNNVectorScoreScript.java	77.77% <100.00%> (ø)	5.00 <0.00> (ø)

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[vamshin]

LGTM! Thanks for the fix.