Make fields protected for DefaultRebalancePreChecker

[somandal]

This PR makes the executorService and pinotHelixResourceManager fields of DefaultRebalancePreChecker class as protected

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.39%. Comparing base (59551e4) to head (81d5743).
Report is 1738 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #15091      +/-   ##
============================================
+ Coverage     61.75%   63.39%   +1.64%     
- Complexity      207     1482    +1275     
============================================
  Files          2436     2744     +308     
  Lines        133233   154078   +20845     
  Branches      20636    23781    +3145     
============================================
+ Hits          82274    97683   +15409     
- Misses        44911    49017    +4106     
- Partials       6048     7378    +1330     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (+99.99%)	⬆️
integration	100.00% <ø> (+99.99%)	⬆️
integration1	100.00% <ø> (+99.99%)	⬆️
integration2	0.00% <ø> (ø)
java-11	63.36% <ø> (+1.65%)	⬆️
java-21	63.27% <ø> (+1.64%)	⬆️
skip-bytebuffers-false	63.38% <ø> (+1.63%)	⬆️
skip-bytebuffers-true	63.25% <ø> (+35.52%)	⬆️
temurin	63.39% <ø> (+1.64%)	⬆️
unittests	63.39% <ø> (+1.64%)	⬆️
unittests1	56.06% <ø> (+9.16%)	⬆️
unittests2	33.88% <ø> (+6.14%)	⬆️

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[somandal]

test failure seems transient, passes when i run it locally:

Error:    OfflineClusterIntegrationTest.testStringComparisonInFilter:3494->BaseClusterIntegrationTest.getType:742 » NullPointer Cannot invoke "com.fasterxml.jackson.databind.JsonNode.get(String)" because the return value of "com.fasterxml.jackson.databind.JsonNode.get(String)" is null