Inline CTEs for modification queries

As @tejeswarm noted, Postgres does inline CTEs when the top-level command is modification, whereas Citus skips.

```C
Postgres

SS_process_ctes
    CmdType         cmdType = ((Query *) cte->ctequery)->commandType;
    if (cmdType == CMD_SELECT && !contain_dml(cte->ctequery) &&....

Citus

InlineCTEsInQueryTree(Query *query)
    PostgreSQLCTEInlineCondition(cte, query->commandType))
        if  cmdType == CMD_SELECT && !contain_dml(cte->ctequery) &&...
```

We should be careful about stable functions inside CTEs.