Add pg_stat_vfdcache view for VFD cache statistics

Hello hackers,

This comes from Tomas's patch idea from his website[1], i thought this
patch makes sense to have.

PostgreSQL's virtual file descriptor (VFD) maintains a
per-backend cache of open file descriptors, bounded by
max_files_per_process (default 1000). When the cache is full, the
least-recently-used entry is evicted so its OS fd is closed, so a new
file can be opened. On the next access to that file, open() must be
called again, incurring a syscall that a larger cache would have
avoided.

A trivial example is with partitioned tables: a table with 1500
partitions requires even more than 1500 file descriptors per full scan (main
fork, vm ...), which is more than the default limit, causing potential
evictions and reopens.

The problem is well-understood and the fix is straightforward: raise
max_files_per_process. Tomas showed a 4-5x throughput
improvement in [1] sometimes, on my end i see something less than that,
depending on the query itself, but we get the idea.

AFAIK there is currently no way from inside PostgreSQL to know whether fd
cache pressure is occurring.

Implementation is trivial, because the VFD cache is strictly per-backend,
the counters are also
per-backend and require no shared memory or locking. Three macros
(pgstat_count_vfd_hit/miss/eviction) update fields in PendingVfdCacheStats
directly from fd.c.

I find this a bit useful, I would love to hear about anyone's thoughts
whether this is useful or not.

[1] https://vondra.me/posts/patch-idea-statistics-for-file-descriptor-cache

Regards,
Ayoub