Allow cumulative statistics to read/write auxiliary data from/to disk

Allow cumulative statistics to read/write auxiliary data from/to disk

Cumulative stats kinds gain the capability to write additional per-entry
data when flushing the stats at shutdown, and read this data when
loading back the stats at startup.  This can be fit for example in the
case of variable-length data (like normalized query strings), so as it
becomes possible to link the shared memory stats entries to data that is
stored in a different area, like a DSA segment.

Three new optional callbacks are added to PgStat_KindInfo, available to
variable-numbered stats kinds:
* to_serialized_data: writes auxiliary data for an entry.
* from_serialized_data: reads auxiliary data for an entry.
* finish: performs actions after read/write/discard operations.  This is
invoked after processing all the entries of a kind, allowing extensions
to close file handles and clean up resources.

Stats kinds have the option to store this data in the existing pgstats
file, but can as well store it in one or more additional files whose
names can be built upon the entry keys.  The new serialized callbacks
are called once an entry key is read or written from the main stats
file.  A file descriptor to the main pgstats file is available in the
arguments of the callbacks.

Author: Sami Imseih <samimseih@gmail.com>
Co-authored-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0s9SDOu+Z6veoJCHWk+kDeTktAtC-KY9fQ9Z6BJdDUirQ@mail.gmail.com