Add flatten_result_index_mapping field to Config

[jackiehanyang]

Description

This pr backports #1276 into 2.x branch

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[codecov]

Codecov Report

Attention: Patch coverage is 76.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 77.64%. Comparing base (138dbd0) to head (7073299).
Report is 25 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
.../java/org/opensearch/ad/model/AnomalyDetector.java	75.00%	1 Missing and 1 partial ⚠️
...java/org/opensearch/forecast/model/Forecaster.java	33.33%	2 Missing ⚠️
...n/java/org/opensearch/timeseries/model/Config.java	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #1317      +/-   ##
============================================
+ Coverage     71.40%   77.64%   +6.24%     
- Complexity     4878     5469     +591     
============================================
  Files           518      533      +15     
  Lines         22931    23334     +403     
  Branches       2260     2311      +51     
============================================
+ Hits          16373    18118    +1745     
+ Misses         5509     4159    -1350     
- Partials       1049     1057       +8     

Flag	Coverage Δ
plugin	77.64% <76.00%> (+6.24%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/main/java/org/opensearch/ad/model/ADTask.java	98.00% <100.00%> (+<0.01%)	⬆️
.../handler/AbstractAnomalyDetectorActionHandler.java	100.00% <100.00%> (+80.00%)	⬆️
...va/org/opensearch/forecast/model/ForecastTask.java	97.99% <100.00%> (+0.41%)	⬆️
.../rest/handler/AbstractForecasterActionHandler.java	91.11% <100.00%> (+91.11%)	⬆️
.../java/org/opensearch/ad/model/AnomalyDetector.java	86.08% <75.00%> (-0.95%)	⬇️
...java/org/opensearch/forecast/model/Forecaster.java	78.10% <33.33%> (+7.02%)	⬆️
...n/java/org/opensearch/timeseries/model/Config.java	85.30% <66.66%> (-1.72%)	⬇️

... and 110 files with indirect coverage changes