Allow usage of `zerolog.Ctx(ctx)` in `Enabled()` and `Handle()` methods

[dhermes]

It's possible to attach a zerolog logger to a root context

logger := zerolog.New(...)
ctx = logger.WithContext(ctx)

and then later access it via zerolog.Ctx(ctx).

This is a fairly common pattern (attaching a logger to + reading a logger from a context). An example implementation of Enabled() with it could be:

// Enabled checks if the given level is enabled for the logger.
func (*ZerologHandler) Enabled(ctx context.Context, level slog.Level) bool {
	zl := zerolog.Ctx(ctx)
	switch level {
	case slog.LevelDebug:
		return zl.Debug().Enabled()
	case slog.LevelInfo:
		return zl.Info().Enabled()
	case slog.LevelWarn:
		return zl.Warn().Enabled()
	case slog.LevelError:
		return zl.Error().Enabled()
	default:
		return false
	}
}

(The above is not intended to cover fallbacks / overrides / alternative resolution methods, just a particular way to use zerolog.Ctx(ctx). Note also you could use zerolog.Ctx(ctx).GetLevel() if the above implementation is not adequate.)