Fix const correctness in pgstat data serialization callbacks

Fix const correctness in pgstat data serialization callbacks

4ba012a8ed9c defined the "header" (pointer to the stats data) of
from_serialized_data() as a const, even though it is fine (and
expected!) for the callback to modify the shared memory entry when
loading the stats at startup.

While on it, this commit updates the callback to_serialized_data() in
the test module test_custom_stats to make the data extracted from the
"header" parameter a const since it should never be modified: the stats
are written to disk and no modifications are expected in the shared
memory entry.

This clarifies the API contract of these new callbacks.

Reported-By: Peter Eisentraut <[email protected]>
Author: Michael Paquier <[email protected]>
Co-authored-by: Sami Imseih <[email protected]>
Discussion: https://postgr.es/m/d87a93b0-19c7-4db6-b9c0-d6827e7b2da1@eisentraut.org