Implement OR REPLACE option for CREATE AGGREGATE.

Aggregates have acquired a dozen or so optional attributes in recent
years for things like parallel query and moving-aggregate mode; the
lack of an OR REPLACE option to add or change these for an existing
agg makes extension upgrades gratuitously hard. Rectify.

src/backend/nodes/copyfuncs.c

@@ -3372,6 +3372,7 @@ _copyDefineStmt(const DefineStmt *from)
 	COPY_NODE_FIELD(args);
 	COPY_NODE_FIELD(definition);
 	COPY_SCALAR_FIELD(if_not_exists);
+	COPY_SCALAR_FIELD(replace);
 
 	return newnode;
 }