injection points for hash aggregation

Attached is a patch that adds a few injection points for hash
aggregation.

A couple questions on the injection points framework:

* The patch allows forcing the partition fan-out to one. I could
imagine forcing it to a specific value, is there a way to do that?

* The injection_points extension offers the concept of a "local"
injection point. While that makes sense for the callback function, it
doesn't make sense when using the "local_var = 123" style, because that
will happen regardless of the condition, right?

* Callbacks are given very little context from the injection point
itself, so it's hard for me to imagine what a callback might do other
than logging the name of the injection point or waiting (as the
extension implements). What else would callbacks be good for?

-- 
Jeff Davis
PostgreSQL Contributor Team - AWS