plpgsql_check can detect bad default volatility flag

Common performance problem of plpgsql function when these functions are used from some more complex queries is using default VOLATILE flag. There are not possible to do more aggressive optimization of this function call. plpgsql_check can detect this issue now:

CREATE OR REPLACE FUNCTION public.flag_test1(integer)
 RETURNS integer
 LANGUAGE plpgsql
 STABLE
AS $function$
begin
  return $1 + 10;
end;
$function$;

CREATE OR REPLACE FUNCTION public.flag_test2(integer)
 RETURNS integer
 LANGUAGE plpgsql
 VOLATILE
AS $function$
begin
  return (select * from fufu where a = $1 limit 1);
end;
$function$;

postgres=# select * from plpgsql_check_function('flag_test1(int)', performance_warnings => true);
┌────────────────────────────────────────────────────────────────────┐
│                       plpgsql_check_function                       │
╞════════════════════════════════════════════════════════════════════╡
│ performance:00000:routine is marked as STABLE, should be IMMUTABLE │
└────────────────────────────────────────────────────────────────────┘
(1 row)

postgres=# select * from plpgsql_check_function('flag_test2(int)', performance_warnings => true);
┌──────────────────────────────────────────────────────────────────────┐
│                        plpgsql_check_function                        │
╞══════════════════════════════════════════════════════════════════════╡
│ performance:00000:routine is marked as VOLATILE, should be STABLE    │
└──────────────────────────────────────────────────────────────────────┘
(1 row)