Accept FetchedValue, text() for column "default" value

Accept FetchedValue, text() for column "default" value

Fixed bug whereby the :meth:`.DDLEvents.column_reflect` event would not
allow a non-textual expression to be passed as the value of the
"default" for the new column, such as a :class:`.FetchedValue`
object to indicate a generic triggered default or a
:func:`.sql.expression.text` construct.  Clarified the documentation
in this regard as well.

Fixes: #3905
Change-Id: I829796c3e9f87f375149bebee7eef133a6876d4d