Change publication's publish_generated_columns option type to enum.

Change publication's publish_generated_columns option type to enum.

The current boolean publish_generated_columns option only supports a
binary choice, which is insufficient for future enhancements where
generated columns can be of different types (e.g., stored or virtual). The
supported values for the publish_generated_columns option are 'none' and
'stored'.

Author: Vignesh C 
Reviewed-by: Peter Smith 
Reviewed-by: Peter Eisentraut 
Reviewed-by: Amit Kapila 
Discussion: https://postgr.es/m/d718d219-dd47-4a33-bb97-56e8fc4da994@eisentraut.org
Discussion: https://postgr.es/m/B80D17B2-2C8E-4C7D-87F2-E5B4BE3C069E@gmail.com