pgsql: Add information about access method for partitioned relations in

Add information about access method for partitioned relations in \dP+

Since 374c7a229042, it is possible to set a table AM on a partitioned
table.  This information was showing up already in psql with \d+, while
\dP+ provided no information.

This commit extends \dP+ to show the access method used by a partitioned
table or index, if set.

Author: Justin Pryzby
Discussion: https://postgr.es/m/ZkyivySXnbvOogZz@pryzbyj2023

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/978f38c771fb3a19fdd5cb73cb662441eb9e551c

Modified Files
--------------
src/bin/psql/describe.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)