Refactor logging to use slog package across the application

- Replaced standard log package with log/slog in httpclient, nginx manager, sync service, updater, and other components for structured logging.
- Introduced environment variable `LOG_LEVEL` to control logging levels (debug, info, warn, error).
- Updated documentation to reflect changes in logging configuration and requirements.
- Added a new logging setup function in the ats_agent internal package to initialize the slog logger.

atsf_agent/internal/logging/logger.go

@@ -0,0 +1,27 @@
+package logging
+
+import (
+	"log/slog"
+	"os"
+	"strings"
+)
+
+func Setup() {
+	handler := slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{
+		Level: parseLevel(os.Getenv("LOG_LEVEL")),
+	})
+	slog.SetDefault(slog.New(handler))
+}
+
+func parseLevel(value string) slog.Level {
+	switch strings.ToLower(strings.TrimSpace(value)) {
+	case "debug":
+		return slog.LevelDebug
+	case "warn", "warning":
+		return slog.LevelWarn
+	case "error":
+		return slog.LevelError
+	default:
+		return slog.LevelInfo
+	}
+}