"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. We usually break the lines around the 80th column, even if the sentence is finished before that.
2. We try to use punctuation correctly (e.g. "insert, delete,  and update" instead of "insert. delete and update".
3. Should we mention that this function throws an error if the command is not supported?

```C
/*
 * CStorePlanForeignModify checks if the modify operation is supported, and
 * raises an error if the operation is not supported. Only insert command
 * with subquery (i.e. INSERT INTO <table> SELECT ...) is supported. Other forms
 * of insert, delete, and update commands are not supported.
 */
 ```
For example: