detect/transform: Add transform "validate" function

detect/transform: Add transform "validate" function

This commit adds an (optional) entry for a validation function. The
validation function, if present, will be used during rule processing.

Its role is to determine if the arguments are compatible with the
transform. E.g., a content string of "this string has whitespace" is not
compatible with the `strip_whitespace` transform.

(cherry picked from commit 596875fa68af18746abbbd9e307d14a93f23c3f7)