chore: add normalizeStringFlag helper func, improve flag handling

intended for use with flags that accept a custom string input, but which
are used/mapped to a specific value. there are lots of flags which do
not need this normalize because they are _intended_ to be accepted and
used literally (inventory path, hostname, reload interval, etc). but the
flags here accept a custom string input which maps to a log level or log
format, respectively. adding this func standardizes that normalization.

cmd/mango/mango.go

@@ -460,7 +460,7 @@ func main() {
 	}
 
 	// parse log level from flag
-	logLevelFlagVal := strings.TrimSpace(strings.ToLower(viper.GetString("logging.level")))
+	logLevelFlagVal := normalizeStringFlag(viper.GetString("logging.level"))
 	switch logLevelFlagVal {
 	case "info": // default is info, we're good
 	case "warn":
@@ -485,7 +485,7 @@ func main() {
 	metricMangoRuntimeInfo.With(metricMangoRuntimeInfoLabels).Set(1)
 
 	// parse log output format from flag
-	logOutputFormat := strings.TrimSpace(strings.ToLower(viper.GetString("logging.output")))
+	logOutputFormat := normalizeStringFlag(viper.GetString("logging.output"))
 	if logOutputFormat == "json" {
 		jsonLogHandler := slog.NewJSONHandler(os.Stdout, logHandlerOpts)
 		logger = slog.New(jsonLogHandler)
@@ -543,3 +543,7 @@ func main() {
 	// run mango daemon
 	mango(rootCtx, mainLogger, inventoryPath, me)
 }
+
+func normalizeStringFlag(s string) string {
+	return strings.TrimSpace(strings.ToLower(s))
+}