Cannot Use Postgres CURRENT_TIMESTAMP for Column Default

# About

When trying to use `CURRENT_TIMESTAMP` as a column default, the Hasura Console errored out. This is a valid function in PG so not sure why it is blocked. I've used `clock_timestamp()` to work around.

https://www.postgresqltutorial.com/postgresql-current_timestamp/

## Steps To Reproduce

* Go to any table with a timestamp with timezone data type (e.g. updated_at or created_at)
* Edit and enter `CURRENT_TIMESTAMP` for default
* Hit Save

## Expected Behavior

Change to be saved

## Actual Behavior


Reproduce the failure in the Hasura Console using a timestamp-with-timezone column and CURRENT_TIMESTAMP as the default. Trace the Console's handling of the column default, then verify that CURRENT_TIMESTAMP is accepted and the change can be saved without affecting the existing clock_timestamp() workaround.

Written by the indexing model from the issue text.