Validation for enum values

[florinutz]

This adds the is_in_enum validation option for enums, which checks if incoming values are actually defined in the enum. The previous way of doing this was hackish, as one had to use gt and lt in order to check for the range (instead of actual enum values), and the lt would have to be kept updated as more fields are added to the enum.

[fho]

@mwitkow is there any chance to get the PR merged to upstream?

[mwitkow]

Makes sense :) thanks!