Add seq_sync_error_count to subscription statistics.

Add seq_sync_error_count to subscription statistics.

This commit adds a new column, seq_sync_error_count, to the
pg_stat_subscription_stats view. This counter tracks the number of errors
encountered by the sequence synchronization worker during operation.

Since a single worker handles the synchronization of all sequences, this
value may reflect errors from multiple sequences. This addition improves
observability of sequence synchronization behavior and helps monitor
potential issues during replication.

Author: Vignesh C <vignesh21@gmail.com>
Reviewed-by: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/CAA4eK1LC+KJiAkSrpE_NwvNdidw9F2os7GERUeSxSKv71gXysQ@mail.gmail.com