Support Enums within `Schema` or `col_vals_in_set`

[emrynHofmannElephant]

Prework

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Proposal

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Is there any chance we'd be able to have Enum support for either/both the Schema & col_vals_in_set(). It is currently partially supported in the Schema by checking if a column is of type Enum, but nothing more. Similarly within col_vals_in_set() you can hack your way around it by using set=[e.name for e in CustomEnum].

I feel that for data validation it is common to need to check if elements are in a set of values (hence the current function), and Enums are a clean way of storing this information.