Built-in support for Mailgun

[sauloperez]

Is your feature request related to a problem? Please describe.
At Tacto we are looking for a drop-in replacement of django-anymail but in FastAPI, that supports Mailgun, our third-party service. We're migration from Django to FastAPI for our API.

Describe the solution you'd like
Ideally, we want to be able to swap backends, like SMTP or Mailgun, but we might as well switch to a different provider in the future. So we want this to be easy.

What I like is that this project supports backends. However, I wonder what are your thoughts on supporting third-party backends, the same way django-anymail does. We oould provide an implementation of a Mailgun backend, heavily based on https://github.com/anymail/django-anymail/blob/main/anymail/backends/mailgun.py.

Describe alternatives you've considered
Alternatively, if you want to keep this project provider-agnostic, we could implement the Mailgun backend as a separate plugin. We would need a way to support external plugins.

Additional context
Add any other context or screenshots about the feature request here.

[marktennyson]

At the next release I will add a feature to add custom backend plugins with Fastapi-Mailman. Thanks for checking this out.