[Refactor] SegmentsStats#filesSizes from ImmutableOpenMap to java.util.Map

[nknize]

Using an ImmutableOpenMap in o.o.index.engine.SegmentsStats#filesSizes is overkill since all we care about is public facing API not leaking the filesSizes collection. To remove all of the unnecessary temporary object copies saddled with using ImmutableOpenMap internally, this PR refactors the SegmentsStats#filesSizes variable from an ImmutableOpenMap to an unmodifiable java.util.Map. It also refactors the internal only used FILE_DESCRIPTIONS static variable to an unmodifiable java.util.Map in order to completely remove SegmentsStats dependency on hppc based ImmutableOpenMap.

relates #5910

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/13850/
• CommitID: fce11ab
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[dreamer-89]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/13850/
• CommitID: fce11ab
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

Doc count assertion failure, seems non-related to changes. Retrying check.

REPRODUCE WITH: ./gradlew ':modules:geo:internalClusterTest' --tests "org.opensearch.geo.search.aggregations.bucket.GeoTileGridIT.testMultivaluedGeoPointsAggregation" -Dtests.seed=73A6EB980B2F13B4 -Dtests.security.manager=true -Dtests.jvm.argline="-XX:TieredStopAtLevel=1 -XX:ReservedCodeCacheSize=64m" -Dtests.locale=is -Dtests.timezone=Africa/Addis_Ababa -Druntime.java=19

org.opensearch.geo.search.aggregations.bucket.GeoTileGridIT > testMultivaluedGeoPointsAggregation FAILED
    java.lang.AssertionError: Geohash 13/1627/7465 has wrong doc count  expected:<0> but was:<1>
        at __randomizedtesting.SeedInfo.seed([73A6EB980B2F13B4:BE14626187F3995E]:0)
        at org.junit.Assert.fail(Assert.java:89)
        at org.junit.Assert.failNotEquals(Assert.java:835)
        at org.junit.Assert.assertEquals(Assert.java:647)
        at org.opensearch.geo.search.aggregations.bucket.GeoTileGridIT.testMultivaluedGeoPointsAggregation(GeoTileGridIT.java:128)

[reta]

Nice (Spring?) cleanup :)

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.index.WaitUntilRefreshIT.classMethod

• URL: https://build.ci.opensearch.org/job/gradle-check/13858/
• CommitID: fce11ab
  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-commenter]

Codecov Report

Merging #7094 (fce11ab) into main (64f3123) will decrease coverage by 0.30%.
The diff coverage is 75.00%.

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@@             Coverage Diff              @@
##               main    #7094      +/-   ##
============================================
- Coverage     71.10%   70.80%   -0.30%     
+ Complexity    59808    59485     -323     
============================================
  Files          4852     4852              
  Lines        285138   285123      -15     
  Branches      41107    41104       -3     
============================================
- Hits         202755   201892     -863     
- Misses        66040    66628     +588     
- Partials      16343    16603     +260     

Impacted Files	Coverage Δ
.../main/java/org/opensearch/index/engine/Engine.java	74.09% <40.00%> (-2.06%)	⬇️
...ava/org/opensearch/index/engine/SegmentsStats.java	72.41% <80.00%> (+1.42%)	⬆️

... and 446 files with indirect coverage changes

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[nknize]

I opened #7101 for the unrelated GeoHashGridIT failure. Rerunning the cancelled precommit job and will merge on green.