(#1506) Remove reservedSymbolTable and replace with HIDDEN_FIELD_NAME

[acarbonetto]

Description

The reservedSymbolTable was included to separate out fields from hidden fields. It makes more sense to use the HIDDEN_FIELD_NAME namespace instead. Nothing changes for down-stream calls.

Issues Resolved

Fixes #1506

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

Merging #1936 (b410e8d) into main (245c4f8) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main    #1936      +/-   ##
============================================
- Coverage     97.51%   97.51%   -0.01%     
+ Complexity     4657     4656       -1     
============================================
  Files           408      408              
  Lines         11933    11930       -3     
  Branches        829      829              
============================================
- Hits          11637    11634       -3     
  Misses          289      289              
  Partials          7        7              

Flag	Coverage Δ
sql-engine	97.51% <100.00%> (-0.01%)	⬇️

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

Files Changed	Coverage Δ
...a/org/opensearch/sql/analysis/TypeEnvironment.java	100.00% <ø> (ø)
...ain/java/org/opensearch/sql/analysis/Analyzer.java	100.00% <100.00%> (ø)
...rg/opensearch/sql/analysis/ExpressionAnalyzer.java	100.00% <100.00%> (ø)
.../org/opensearch/sql/analysis/symbol/Namespace.java	100.00% <100.00%> (ø)

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/sql/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/sql/backport-2.x
# Create a new branch
git switch --create backport/backport-1936-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 5381a6f76ddc73c0cb14ad5befc444ec68dfdaed
# Push it to GitHub
git push --set-upstream origin backport/backport-1936-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/sql/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-1936-to-2.x.