Allow calling generate_uid on before_validation

It would be nice to be able to set validations on the unique id column. Letting someone set the callback to be `before_validation` instead of `before_create` means you are guaranteed to have the unique ID set so that validations can be run. Likewise, it makes it easier to backfill or update records that already exist since you can simply set the column to nil before saving to update it.

Right now since it is on `before_create` there is no way to have validations on the column.