timer: add new feature FixedRandomDelay=

FixedRandomDelay=yes will use
`siphash24(sd_id128_get_machine() || MANAGER_IS_SYSTEM(m) || getuid() || u->id)`,
where || is concatenation, instead of a random number to choose a value between
0 and RandomizedDelaySec= as the timer delay.
This essentially sets up a fixed, but seemingly random, offset for each timer
iteration rather than having a random offset recalculated each time it fires.

Closes #10355

Co-author: Anita Zhang <the.anitazha@gmail.com>

src/shared/bus-unit-util.c

@@ -2074,7 +2074,8 @@ static int bus_append_timer_property(sd_bus_message *m, const char *field, const
                               "RemainAfterElapse",
                               "Persistent",
                               "OnTimezoneChange",
-                              "OnClockChange"))
+                              "OnClockChange",
+                              "FixedRandomDelay"))
                 return bus_append_parse_boolean(m, field, eq);
 
         if (STR_IN_SET(field, "AccuracySec",