Add pg_buffercache_usage_counts() to contrib/pg_buffercache.

Add pg_buffercache_usage_counts() to contrib/pg_buffercache.

It was pointed out that pg_buffercache_summary()'s report of
the overall average usage count isn't that useful, and what
would be more helpful in many cases is to report totals for
each possible usage count.  Add a new function to do it like
that.  Since pg_buffercache 1.4 is already new for v16,
we don't need to create a new extension version; we'll just
define this as part of 1.4.

Nathan Bossart

Discussion: https://postgr.es/m/20230130233040.GA2800702@nathanxps13