Support for nested expression serialization

[MaxKsyunz]

Description

Work-in-progress that removes double-base64 encoding of serialized physical plans. This was increasing size unnecessarily.

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[codecov]

Codecov Report

Merging #249 (ba9130a) into dev-pagination-v2_revC (ec5fb40) will decrease coverage by 0.26%.
The diff coverage is 96.77%.

@@                     Coverage Diff                      @@
##             dev-pagination-v2_revC     #249      +/-   ##
============================================================
- Coverage                     98.44%   98.18%   -0.26%     
+ Complexity                     3844     3841       -3     
============================================================
  Files                           359      360       +1     
  Lines                          9431     9439       +8     
  Branches                        615      615              
============================================================
- Hits                           9284     9268      -16     
- Misses                          142      164      +22     
- Partials                          5        7       +2     

Flag	Coverage Δ
sql-engine	98.18% <96.77%> (-0.26%)	⬇️

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Impacted Files	Coverage Δ
...rg/opensearch/sql/executor/PaginatedPlanCache.java	77.27% <90.00%> (-22.73%)	⬇️
...ion/serialization/DefaultExpressionSerializer.java	100.00% <100.00%> (ø)
...on/serialization/NoEncodeExpressionSerializer.java	100.00% <100.00%> (ø)
...ensearch/sql/planner/physical/ProjectOperator.java	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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