Add feature: listKeys Scalar Transformation Function

[Navya-Kumar]

add listKeys() scalar transformation function and unit tests under new mapfunctions class. takes in JSON map input and outputs string array of all keys in map.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.96721% with 11 lines in your changes missing coverage. Please review.

Project coverage is 61.96%. Comparing base (59551e4) to head (1c3c605). Report is 895 commits behind head on master.

Files	Patch %	Lines
...che/pinot/common/function/scalar/MapFunctions.java	81.96%	9 Missing and 2 partials :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #13581      +/-   ##
============================================
+ Coverage     61.75%   61.96%   +0.21%     
+ Complexity      207      198       -9     
============================================
  Files          2436     2554     +118     
  Lines        133233   140532    +7299     
  Branches      20636    21857    +1221     
============================================
+ Hits          82274    87083    +4809     
- Misses        44911    46824    +1913     
- Partials       6048     6625     +577     

Flag	Coverage Δ
custom-integration1	<0.01% <0.00%> (-0.01%)	:arrow_down:
integration	<0.01% <0.00%> (-0.01%)	:arrow_down:
integration1	<0.01% <0.00%> (-0.01%)	:arrow_down:
integration2	0.00% <0.00%> (ø)
java-11	61.93% <81.96%> (+0.22%)	:arrow_up:
java-21	61.84% <81.96%> (+0.22%)	:arrow_up:
skip-bytebuffers-false	61.95% <81.96%> (+0.20%)	:arrow_up:
skip-bytebuffers-true	61.82% <81.96%> (+34.09%)	:arrow_up:
temurin	61.96% <81.96%> (+0.21%)	:arrow_up:
unittests	61.96% <81.96%> (+0.21%)	:arrow_up:
unittests1	46.46% <81.96%> (-0.43%)	:arrow_down:
unittests2	27.72% <0.00%> (-0.02%)	:arrow_down:

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