Adding new output parameter of pg_stat_statements to identify operation of the query.

Hi PG hackers:
When using pg_stat_statements to collect running SQL of PG, we find it is hard for our program to get exact operation type of the SQL, such as SELECT, DELETE, UPDATE, INSERT, and so on.
So we modify the the source code of pg_stat_statements and add another output parameter to tell us the operation type.
Of course some application know their operation type, but for us and many public databases, doing this is hard.
The only way is to reparse the SQL, obviously it is too expensive for a monitoring or diagnosis system.
We have done the job and are willing to post a patch.

jasonysli