[Backport 2.x] Datetime aggregation fixes.

[opensearch-trigger-bot]

Backport 5220a98 from #1061

[codecov-commenter]

Codecov Report

Merging #1312 (71346f6) into 2.x (b62c55c) will increase coverage by 0.01%.
The diff coverage is 100.00%.

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@@             Coverage Diff              @@
##                2.x    #1312      +/-   ##
============================================
+ Coverage     98.35%   98.37%   +0.01%     
- Complexity     3632     3645      +13     
============================================
  Files           345      345              
  Lines          8957     9022      +65     
  Branches        578      585       +7     
============================================
+ Hits           8810     8875      +65     
  Misses          142      142              
  Partials          5        5              

Flag	Coverage Δ
sql-engine	98.37% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
.../org/opensearch/sql/data/model/ExprValueUtils.java	100.00% <ø> (ø)
...sql/expression/aggregation/AggregatorFunction.java	100.00% <100.00%> (ø)
...arch/sql/expression/aggregation/AvgAggregator.java	100.00% <100.00%> (ø)
...cript/aggregation/ExpressionAggregationScript.java	100.00% <100.00%> (ø)
...ript/aggregation/dsl/BucketAggregationBuilder.java	100.00% <100.00%> (ø)

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