Support opfamily members in get_object_address

In the spirit of 890192e99a and 4464303405: have get_object_address
understand individual pg_amop and pg_amproc objects.  There is no way to
refer to such objects directly in the grammar -- rather, they are almost
always considered an integral part of the opfamily that contains them.
(The only case that deals with them individually is ALTER OPERATOR
FAMILY ADD/DROP, which carries the opfamily address separately and thus
does not need it to be part of each added/dropped element's address.)
In event triggers it becomes possible to become involved with individual
amop/amproc elements, and this commit enables pg_get_object_address to
do so as well.

To make the overall coding simpler, this commit also slightly changes
the get_object_address representation for opclasses and opfamilies:
instead of having the AM name in the objargs array, I moved it as the
first element of the objnames array.  This enables the new code to use
objargs for the type names used by pg_amop and pg_amproc.

Reviewed by: Stephen Frost

src/backend/commands/event_trigger.c

@@ -1060,6 +1060,8 @@ EventTriggerSupportsObjectType(ObjectType obtype)
 			/* no support for event triggers on event triggers */
 			return false;
 		case OBJECT_AGGREGATE:
+		case OBJECT_AMOP:
+		case OBJECT_AMPROC:
 		case OBJECT_ATTRIBUTE:
 		case OBJECT_CAST:
 		case OBJECT_COLUMN: