Allow to store and retrieve logger in `context.Context` object

[cardil]

The standard Golang context.Context allows storing and retrieve objects using WithValue and Value func.

It would be a good idea to create a common API to configure slf4go using context.Context API, apart from existing usage of global variable. The current approach of using global variables (globalLogger, and globalDriver) are not always acceptable in some projects, as it's susceptible to race conditions to occur.

An example of a new API:

// configure logger: driver, level, etc. withing context
d := slogzap.Driver{/* ... */}
ctx = slog.WithLogger(ctx, slog.Config{Driver: d})

// usage
func someUserFunc(ctx context.Context) {
  log := slog.FromContext(ctx)
  log.Info("Example msg")
}