Fix access specifier for FieldMapper method to allow usage by plugins

[mgodwan]

Description

When enabling scaled_float field, due to different ClassLoader being used for module, and core, ScaledFloatFieldMapper is unable to access package-protected methods. This is leading to process being killed due to the fatal error. This commit fixes the same.

java.lang.IllegalAccessError: class org.opensearch.index.mapper.ScaledFloatFieldMapper tried to access method 'void org.opensearch.index.mapper.FieldMapper.checkStoredAndDocValuesForDerivedSource()' (org.opensearch.index.mapper.ScaledFloatFieldMapper is in unnamed module of loader java.net.FactoryURLClassLoader @16b57f2; org.opensearch.index.mapper.FieldMapper is in unnamed module of loader 'app')

Related Issues

NA

Check List

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

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[mgodwan]

Note: The issue only impacts when derived source is enabled on an index with scaled_float mapping type.

[github-actions]

✅ Gradle check result for 0ecc15f: SUCCESS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.81%. Comparing base (c442cce) to head (0ecc15f).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19113      +/-   ##
============================================
- Coverage     72.98%   72.81%   -0.18%     
+ Complexity    69475    69388      -87     
============================================
  Files          5647     5647              
  Lines        319210   319210              
  Branches      46171    46171              
============================================
- Hits         232977   232420     -557     
- Misses        67385    67986     +601     
+ Partials      18848    18804      -44     

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