Add target "slru" to pg_stat_reset_shared()

Add target "slru" to pg_stat_reset_shared()

Currently, pg_stat_reset_shared() cannot reset the counters in the view
pg_stat_slru even if it is a type of shared stats.  This patch adds
support for a new value in pg_stat_reset_shared(), called "slru", able
to do that.  Note that pg_stat_reset_shared(NULL) also resets SLRU
counters.

There may be a point in removing pg_stat_reset_slru() that was
introduced in 28cac71bd368 (v13~) as the new option overlaps with this
function, but we would lose the ability to reset individual SLRU
counters.  This is left for future reconsideration.

Author: Atsushi Torikoshi
Discussion: https://postgr.es/m/e3c25d72e81378e7b64f3c52e0306fc9@oss.nttdata.com