Invent pgstat_fetch_stat_backend_by_pid()

Invent pgstat_fetch_stat_backend_by_pid()

This code is extracted from pg_stat_get_backend_io() in pgstatfuncs.c,
so as it can be shared with other areas that need backend pgstats
entries while having the benefits of the various sanity checks
refactored here.  As per its name, this retrieves backend statistics
based on a PID, with the option of retrieving a BackendType if given in
input.

Currently, this is used for the backend-level IO statistics.  The next
move would be to reuse that for the backend-level WAL statistics.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/Z3zqc4o09dM/Ezyz@ip-10-97-1-34.eu-west-3.compute.internal