✨ Support firebase FCM as a notify subscriber

pkg/external/firebase.go

@@ -0,0 +1,21 @@
+package external
+
+import (
+	"context"
+	firebase "firebase.google.com/go"
+	"google.golang.org/api/option"
+)
+
+var Fire *firebase.App
+
+func SetupFirebase(credentials string) error {
+	opt := option.WithCredentialsFile(credentials)
+	app, err := firebase.NewApp(context.Background(), nil, opt)
+	if err != nil {
+		return err
+	} else {
+		Fire = app
+	}
+
+	return nil
+}