Add support for DEFAULT in ALTER TABLE .. SET ACCESS METHOD

Add support for DEFAULT in ALTER TABLE .. SET ACCESS METHOD

This option can be used to switch a relation to use the access method
set by default_table_access_method when running the command.

This has come up when discussing the possibility to support setting
pg_class.relam for partitioned tables (left out here as future work),
while being useful on its own for relations with physical storage as
these must have an access method set.

Per suggestion from Justin Pryzby.

Author: Michael Paquier
Reviewed-by: Justin Pryzby
Discussion: https://postgr.es/m/ZeCZ89xAVFeOmrQC@pryzbyj2023