refactor(sol): reduce local variables in SortMergeJoinExec

[stuarttimwhite]

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Rationale for this change

The proof of sql contracts repo needs there to be fewer local variables in SortMergeJoinExec.

What changes are included in this PR?

Adding s struct-like object within SortMergeJoinExec to reduce local variables.

Are these changes tested?

Yes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.45%. Comparing base (9337aef) to head (4312e52).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1107      +/-   ##
==========================================
- Coverage   96.77%   96.45%   -0.33%     
==========================================
  Files         302      293       -9     
  Lines       59220    52149    -7071     
==========================================
- Hits        57312    50301    -7011     
+ Misses       1908     1848      -60     

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[github-actions]

🎉 This PR is included in version 0.127.24 🎉

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