fix: Allow specify scalar index type

[congqixia]

Related to #671

[sre-ci-robot]

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[congqixia]

/kind bug

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.91%. Comparing base (3eabbb5) to head (c1579b9).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #691      +/-   ##
==========================================
+ Coverage   71.81%   71.91%   +0.09%     
==========================================
  Files          51       51              
  Lines        4748     4750       +2     
==========================================
+ Hits         3410     3416       +6     
+ Misses       1214     1210       -4     
  Partials      124      124              

Files	Coverage Δ
entity/index.go	100.00% <ø> (ø)
entity/index_scalar.go	85.71% <100.00%> (+85.71%)	⬆️

[weiliu1031]

/lgtm