♻️ add `logs` configuration settings and replace logging system by `spdlog`

Adds configuration keys to tweak the logger behavior:

{
  "logs": {
     "filepath": "/path/to/log_file",
     "log_level": "warn",
     "max_files": 3,
     "max_size": 100
  }
}

There is now a simple way to log stuff and an interface to retrieve a registered logger. These can be retrieved from anywhere in the program using spdlog::get(logger_name) that returns a shared pointer to the logger.

auto logger = spdlog::get("switcher");
logger->info("Cocorico!");

2021-11-25 12:34:56.808|default|4264f281-9e77-41d0-aee9-b435954c44de|126238|126238|info: Cocorico!|:

Note that this would not show the basename of the source file, nor the line number where the logger function was called.

To show basename and line number in logs, use this instead:

auto logger = spdlog::get("switcher");
LOGGER_INFO(logger, "Cocorico! {} {:d}", "Pain au Chocolat", 42);

2021-11-25 12:34:56.808|default|4264f281-9e77-41d0-aee9-b435954c44de|126238|126238|info: Cocorico! Pain au Chocolat 42|file.cpp:84

The logger first uses a console sink as its main output until the logs settings can be checked, then it takes a file sink in addition to the console sink. This integration respects the previous one as much as possible to limit procedural changes in developers workflows. The previous logging system has been completely removed from the project and with it the many ways to use it.

Each instance of Switcher use a unique global logger but has its own sinks identified by the instance name and uuid.

Here's a snapshot of what logging looks like now:

Read documentation for the key in Configuration for more