Get default column definition in auto schema mapping

[StrikeW]

          > > And when create a new table, we won't capture the default value definition from upstream

Would this be somehow inconsistent and surprising to users? Note that default values are also used when performing DML.

For the case of auto schema mapping (create table t (*)), we can improve the feature in future to get the default value from upstream.
For the table with column definitions manually specified by user, I think they should also specify the default column by hand in the DDL.

Originally posted by @StrikeW in #18322 (comment)