evalengine: Support built-in MySQL function LOWER()

[Weijun-H]

Signed-off-by: Weijun-H [email protected]

Description

Implement support LOWER()

Related Issue(s)

• evalengine: Support all built-in MySQL functions #9647

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[frouioui]

I think we should use the default collation stored in ExpressionEnv here and in builtinLcase.call too.