Add extension_state member to PlannedStmt.

 Add extension_state member to PlannedStmt.
 
 Extensions can stash data computed at plan time into this list using
 planner_shutdown_hook (or perhaps other mechanisms) and then access
 it from any code that has access to the PlannedStmt (such as explain
 hooks), allowing for extensible debugging and instrumentation of
 plans.
 
 Reviewed-by: Andrei Lepikhov <lepihov@gmail.com>
 Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
 Discussion: http://postgr.es/m/CA+TgmoYWKHU2hKr62Toyzh-kTDEnMDeLw7gkOOnjL-TnOUq0kQ@mail.gmail.com