Use new ScalarValidator for variables CI keyword

What does this MR do and why?

Users have been trying to use the new CI inputs feature to pass boolean values to variables defined using the variables keyword. This has not historically been allowed for reasons that are now lost to time. Product gave us the green light to allow variable values to be any scalar, so this commit adds a new ScalarValidator and uses it with variables.

Changelog: changed

Issue: #444318 (closed)

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