Extract logic filling pg_stat_get_io()'s tuplestore into its own routine

Extract logic filling pg_stat_get_io()'s tuplestore into its own routine

This commit adds pg_stat_io_build_tuples(), a helper routine for
pg_stat_get_io(), that fills its result tuplestore based on the contents
of PgStat_BktypeIO.  This will be used in a follow-up commit that uses
the same structures as pg_stat_io for reporting, including the same
object types and contexts, but for a different statistics kind.

Author: Bertrand Drouvot, Michael Paquier
Discussion: https://postgr.es/m/ZtXR+CtkEVVE/LHF@ip-10-97-1-34.eu-west-3.compute.internal