Allow transfer of PostgreSQL interval data type

[tonkas64]

I have a dataset I need to transfer that includes the Postgres interval dataype, this doesn't appear to be currently supported.

I would like the interval type to be transfered as though it's a string/text type. My target (Redshift) can't store the interval type but can store the text and cast it back to interval.

My naive guess is this would 'just' be a change to discovery_utils.py

if data_type == 'interval':
        schema['type'] = nullable_column('string', col.is_primary_key)
        return schema