test_custom_stats: Add tests with read/write of auxiliary data

test_custom_stats: Add tests with read/write of auxiliary data

This commit builds upon 4ba012a8ed9c, giving an example of what can be
achieved with the new callbacks.  This provides coverage for the new
pgstats APIs, while serving as a reference template.

Note that built-in stats kinds could use them, we just don't have a
use-case there yet.

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