Allocate GUC strings in TopMemoryContext

[pgguru]

Be explicit about using TopMemoryContext for the _PG_init() GUC setup memory context; it looks like there are cases that the strings can be deallocated.

This was showing up in crashes in a SHOW ALL output:

#2  0x0000000000a8e58c in ShowAllGUCConfig (dest=<optimized out>) at utils/misc/guc_funcs.c:510
510                             values[2] = PointerGetDatum(cstring_to_text(conf->short_desc));
(gdb) p conf
$1 = (struct config_generic *) 0x2c98580
(gdb) p *conf
$2 = {name = 0x2c98688 "pg_parquet.enable_copy_hooks", context = PGC_USERSET, group = CUSTOM_OPTIONS,
  short_desc = 0x7f9f3f246120 <error: Cannot access memory at address 0x7f9f3f246120>,
  long_desc = 0x7f9f3f246148 <error: Cannot access memory at address 0x7f9f3f246148>, flags = 0, vartype = PGC_BOOL, status = 0, source = PGC_S_FILE,
  reset_source = PGC_S_FILE, scontext = PGC_POSTMASTER, reset_scontext = PGC_POSTMASTER, srole = 10, reset_srole = 10, stack = 0x0, extra = 0x0, nondef_link = {
    prev = 0x2c5a418, next = 0x2c98c60}, stack_link = {next = 0x0}, report_link = {next = 0x0}, last_reported = 0x0,
  sourcefile = 0x2c986b0 "/var/lib/pgsql/17/data/postgresql.conf", sourceline = 52}
(gdb) p *conf->short_desc
Cannot access memory at address 0x7f9f3f246120

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.30%. Comparing base (dcf6fc0) to head (cb48d8a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #165   +/-   ##
=======================================
  Coverage   91.30%   91.30%           
=======================================
  Files          97       97           
  Lines       11204    11206    +2     
=======================================
+ Hits        10230    10232    +2     
  Misses        974      974           

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