support virtual / computed columns in columnstore

If you declare a computed column in a table then an error is returned stating that these are not supported:
MariaDB [test]> create table test(i1 int, c1 varchar(7), d varchar(5) as (left(C1,5)) virtual) engine=columnstore;
ERROR 1910 (HY000): Columnstore storage engine does not support computed columns
MariaDB [test]> create table test(i1 int, c1 varchar(7), d varchar(5) as (left(C1,5)) persistent) engine=columnstore;
ERROR 1910 (HY000): Columnstore storage engine does not support computed columns
MariaDB [test]>

We should consider supporting these, for more details:
https://mariadb.com/kb/en/mariadb/virtual-computed-columns/