Debug CLI for msg to eml conversion (#4643)

<!-- PR description-->

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [x] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Issue(s)

<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* https://github.com/alcionai/corso/issues/3893

#### Test Plan

<!-- How will this be tested prior to merging.-->
- [ ] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/cmd/converter/converter.go

@@ -0,0 +1,51 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"os"
+
+	"github.com/alcionai/corso/src/internal/converters/eml"
+	"github.com/alcionai/corso/src/pkg/services/m365/api"
+)
+
+func main() {
+	if len(os.Args) == 1 {
+		fmt.Println("Usage: converter <source-format> <target-format> <filename>")
+		os.Exit(1)
+	}
+
+	from := os.Args[1]
+	to := os.Args[2]
+	filename := os.Args[3]
+
+	body, err := os.ReadFile(filename)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	var out string
+
+	switch from {
+	case "msg":
+		switch to {
+		case "eml":
+			msg, err := api.BytesToMessageable(body)
+			if err != nil {
+				log.Fatal(err)
+			}
+
+			out, err = eml.ToEml(context.Background(), msg)
+			if err != nil {
+				log.Fatal(err)
+			}
+		default:
+			log.Fatal("Unknown target format", to)
+		}
+	default:
+		log.Fatal("Unknown source format", from)
+	}
+
+	fmt.Print(out)
+}