pg_stat_statements: Add counters for generic and custom plans

pg_stat_statements: Add counters for generic and custom plans

This patch adds two new counters to pg_stat_statements:
- generic_plan_calls
- custom_plan_calls

These counters track how many times a prepared statement was executed
using a generic or custom plan, respectively, providing a global
equivalent at query level, for top and non-top levels, of
pg_prepared_statements whose data is restricted to a single session.

This commit builds upon e125e360020a.  The module is bumped to version
1.13.  PGSS_FILE_HEADER is bumped as well, something that the latest
patches touching the on-disk format of the PGSS file did not actually
bother with since 2022..

Author: Sami Imseih <samimseih@gmail.com>
Reviewed-by: Ilia Evdokimov <ilya.evdokimov@tantorlabs.com>
Reviewed-by: Andrei Lepikhov <lepihov@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Nikolay Samokhvalov <nik@postgres.ai>
Discussion: https://postgr.es/m/CAA5RZ0uFw8Y9GCFvafhC=OA8NnMqVZyzXPfv_EePOt+iv1T-qQ@mail.gmail.com