Add compute_query_id = regress

"regress" is a new mode added to compute_query_id aimed at facilitating
regression testing when a module computing query IDs is loaded into the
backend, like pg_stat_statements.  It works the same way as "auto",
meaning that query IDs are computed if a module enables it, except that
query IDs are hidden in EXPLAIN outputs to ensure regression output
stability.

Like any GUCs of the kind (force_parallel_mode, etc.), this new
configuration can be added to an instance's postgresql.conf, or just
passed down with PGOPTIONS at command level.  compute_query_id uses an
enum for its set of option values, meaning that this addition ensures
ABI compatibility.

Using this new configuration mode allows installcheck-world to pass when
running the tests on an instance with pg_stat_statements enabled,
stabilizing the test output while checking the paths doing query ID
computations.

Reported-by: Anton Melnikov
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/1634283396.372373993@f75.i.mail.ru
Discussion: https://postgr.es/m/YgHlxgc/OimuPYhH@paquier.xyz
Backpatch-through: 14

src/backend/utils/misc/guc.c

@@ -414,6 +414,7 @@ static const struct config_enum_entry backslash_quote_options[] = {
  */
 static const struct config_enum_entry compute_query_id_options[] = {
 	{"auto", COMPUTE_QUERY_ID_AUTO, false},
+	{"regress", COMPUTE_QUERY_ID_REGRESS, false},
 	{"on", COMPUTE_QUERY_ID_ON, false},
 	{"off", COMPUTE_QUERY_ID_OFF, false},
 	{"true", COMPUTE_QUERY_ID_ON, true},