"insert into table select from another table" support

added insert into table support.

This feature is intended to support bulk inserts into cstore table from regular tables. Each insert statement cause creation of a new stripe. Therefore insert command is limited to "insert into <cstore_table> from select * from <another_table>' format. eg. single row inserts like insert into <cstore_table> values (....) is not supported.

Also introduced a regression test for this feature.
1. This comment doesn't completely describe what the function does. (1) It checks if the command is not supported, (2) it returns the list of attributes which we need later in "CStoreBeginForeignModify()".

2. If we are saying what commands are supported and not supported, we should tell it more exactly. Otherwise, the reader may think we are supporting every form of INSERT. We only support commands of form "INSERT INTO ... SELECT ...". Although, if we have already mentioned this in a comment inside the function body, then I am okay we remove this part from the comment here.