[scudo] Refactor allocator config to support optional flags (#81805)

Instead of explicitly disabling a feature by declaring the variable and
set it to false, this change supports the optional flags. I.e., you can
skip certain flags if you are not using it.

This optional feature supports both forms,
  1. Value: A parameter for a feature. E.g., EnableRandomOffset
  2. Type: A C++ type implementing a feature. E.g., ConditionVariableT

On the other hand, to access the flags will be through one of the
wrappers, BaseConfig/PrimaryConfig/SecondaryConfig/CacheConfig
(CacheConfig is embedded in SecondaryConfig). These wrappers have the
getters to access the value and the type. When adding a new feature, we
need to add it to `allocator_config.def` and mark the new variable with
either *_REQUIRED_* or *_OPTIONAL_* macro so that the accessor will be
generated properly.

In addition, also remove the need of `UseConditionVariable` to flip
on/off of condition variable. Now we only need to define the type of
condition variable.