Fix SQLCastExpression for MySQL

[ptoffy]

Currently SQLCastExpression doesn't work on MySQL because of syntax differences. This fixes it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.25%. Comparing base (8fc37b1) to head (023d203).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #4      +/-   ##
==========================================
+ Coverage   91.17%   91.25%   +0.08%     
==========================================
  Files          62       62              
  Lines         940      949       +9     
==========================================
+ Hits          857      866       +9     
  Misses         83       83              

Files with missing lines	Coverage Δ
...s/SQLKitExtras/Expressions/SQLCastExpression.swift	100.00% <100.00%> (ø)

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