Keep `today` as `today` in column default value

[perguth]

Problem

When selecting today as the default value for a column it sets the default to the current days date.

Proposed solution

When selecting a default value for a column it would be helpful to keep today as meaning "always the date on which the new record will be created" (default CURRENT_DATE in SQL) instead of translating it into the concrete date of today.

Additional context

[pavish]

Supporting dynamic defaults is part of our roadmap. Related to discussion: #2272.