Add TupleDescGetDefault()

This unifies some repetitive code.

Note: I didn't push the "not found" error message into the new
function, even though all existing callers would be able to make use
of it.  Using the existing error handling as-is would probably require
exposing the Relation type via tupdesc.h, which doesn't seem
desirable.  (Or even if we changed it to just report the OID, it would
inject the concept of a relation containing the tuple descriptor into
tupdesc.h, which might be a layering violation.  Perhaps some further
improvements could be considered here separately.)

Discussion: https://www.postgresql.org/message-id/flat/52a125e4-ff9a-95f5-9f61-b87cf447e4da%40eisentraut.org

src/include/access/tupdesc.h

@@ -151,4 +151,6 @@ extern TupleDesc BuildDescForRelation(const List *columns);
 
 extern TupleDesc BuildDescFromLists(const List *names, const List *types, const List *typmods, const List *collations);
 
+extern Node *TupleDescGetDefault(TupleDesc tupdesc, AttrNumber attnum);
+
 #endif							/* TUPDESC_H */