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: 225863909