refactor(cubesql): Remove Extension from LogicalPlanLanguage::CubeScan

[mcheshkov]

Check List

• Tests has been run in packages where changes made if available
• Linter has been run for changed code
• Tests for the changes have been added if not covered yet
• Docs have been added / updated if required

Description of Changes Made (if issue reference is not provided)

Extensions is used meaningfully only for CubeScan, but not for CubeScanWrapper. Both are first-class logical plan nodes for us, so Extension is not helpful in egraph

[codecov]

Codecov Report

Attention: Patch coverage is 93.06931% with 28 lines in your changes missing coverage. Please review.

Project coverage is 82.66%. Comparing base (893004a) to head (d9b07d9).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
...t/cubesql/cubesql/src/compile/rewrite/converter.rs	93.05%	28 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8775      +/-   ##
==========================================
- Coverage   82.67%   82.66%   -0.01%     
==========================================
  Files         217      217              
  Lines       78125    78069      -56     
==========================================
- Hits        64588    64535      -53     
+ Misses      13537    13534       -3     

Flag	Coverage Δ
cubesql	82.66% <93.06%> (-0.01%)	⬇️

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