"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.
Does PostgreSQL call ```CStorePlanForeignModify``` for DELETE and UPDATE commands if we haven't set ```ExecForeignDelete``` and ```ExecForeignInsert``` pointers? If it doesn't then we don't need to do this check and we can go with a simple assert:

```C
Assert(plan->operation == CMD_INSERT);
```