[branch2.0](fix) fix varchar len in ctas

[morningman]

This is only for branch-2.0
related to #21754 and #21302

After this, the length of varchar in ctas will be as follows:

1. If derived from a origin column, eg: create table tbl1 as select col1 from tbl2, the length will be same os the origin column.
2. If derived from a function, eg: create table tbl1 as select func(col1) from tbl2, the length will be 65533.
3. If derived from a constant value, eg: create table tbl1 as select "abc" from tbl2, the length will be 65533.