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

doc/src/sgml/config.sgml

@@ -7680,9 +7680,12 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         method is not acceptable.  In this case, in-core computation
         must be always disabled.
         Valid values are <literal>off</literal> (always disabled),
-        <literal>on</literal> (always enabled) and <literal>auto</literal>,
+        <literal>on</literal> (always enabled), <literal>auto</literal>,
         which lets modules such as <xref linkend="pgstatstatements"/>
-        automatically enable it.
+        automatically enable it, and <literal>regress</literal> which
+        has the same effect as <literal>auto</literal>, except that the
+        query identifier is hidden in the <literal>EXPLAIN</literal> output
+        to facilitate automated regression testing.
         The default is <literal>auto</literal>.
        </para>
        <note>