Allow setting arbitrary SMTP URL instead of requiring Resend

[CarlQLange]

I use PurelyMail for all my emailing and it would be nice to just set an SMTP URL for my apps (and for the platform) to use, instead of being tied to Resend.

[hunvreus]

I definitely need to find a better way to add email support. I don't want to force people having to use Resend.