[Backport 2.x] Support object fields in star-tree index (#16728)

[bharath-techie]

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Description

Backport of #16728 resolving conflicts in CHANGELOG.md

Related Issues

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

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❕ Gradle check result for ad17010: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 10 lines in your changes missing coverage. Please review.

Project coverage is 71.87%. Comparing base (bf42c3e) to head (ad17010).
Report is 3 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...va/org/opensearch/index/mapper/StarTreeMapper.java	72.72%	5 Missing and 1 partial ⚠️
...va/org/opensearch/index/mapper/DocumentParser.java	20.00%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #17130      +/-   ##
============================================
- Coverage     71.96%   71.87%   -0.10%     
- Complexity    65772    65773       +1     
============================================
  Files          5325     5326       +1     
  Lines        306459   306531      +72     
  Branches      44698    44712      +14     
============================================
- Hits         220554   220321     -233     
- Misses        67432    67787     +355     
+ Partials      18473    18423      -50     

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