Extend "ALTER EXTENSION ADD object" to permit "DROP object" as well.

Per discussion, this is something we should have sooner rather than later,
and it doesn't take much additional code to support it.

src/include/nodes/parsenodes.h

@@ -1546,14 +1546,15 @@ typedef struct CreateExtensionStmt
 	List	   *options;		/* List of DefElem nodes */
 } CreateExtensionStmt;
 
-typedef struct AlterExtensionAddStmt
+typedef struct AlterExtensionContentsStmt
 {
 	NodeTag		type;
 	char	   *extname;		/* Extension's name */
+	int			action;			/* +1 = add object, -1 = drop object */
 	ObjectType	objtype;		/* Object's type */
 	List	   *objname;		/* Qualified name of the object */
 	List	   *objargs;		/* Arguments if needed (eg, for functions) */
-} AlterExtensionAddStmt;
+} AlterExtensionContentsStmt;
 
 /* ----------------------
  *		Create/Drop FOREIGN DATA WRAPPER Statements