Adds relay_sink, which just wraps a full featured spdlog::logger.

This can be useful in an application which uses several standalone components (typically DLLs) where each instantiates its own logger to define an application specific log messages "routing".

For example, during an initialization of each DLL, the app can chose to replace the DLLs loggers' sinks with the relay_sink with its own global logger and thus rerouting all log messages from all loggers through its own sinks, without destroying the DLLs' loggers.