Allow plugins to create multiple apps

**Is your feature request related to a problem? Please describe.**
I recently started using [gotify-smtp](https://github.com/tystuyfzand/gotify-smtp) to allow applications that don't natively support Gotify to still notify me. Sadly, everything sent to it enters in the "SMTP" app, making it hard to tell from which app the notification came.

**Describe the solution you'd like**
It would be nice if a plugin could generate multiple apps, so it could scale up like 1 SMTP plugin being useful for multiple SMTP senders.

**Describe alternatives you've considered**
The ability to run the same plugin multiple times would also make that possible, if the app name could be configured per plugin. There would however also need to be a way to prevent multiple instances of the same plugin from binding to the same path (in the SMTP plugin's case, port 1025), so this would seem more complex and harder to scale to me.