roll-forward (again) after fixing the windows failure:

    Introduce a logger overriding mechanism.

    Before: the whole program has to link against a unique
    Logger::Singleton(), and there is no default. It's hard to control when
    to use the custom logger vs the default.

    After: By default always use the default logger. The pubsub logger
    overrides via REGISTER_MODULE_INITIALIZER. Multiple implementations can
    co-exist.

    This simplifies the registration management and dependency management.

PiperOrigin-RevId: 226065246