mail_tracking_mailgun: Enable option to send emails via API

[JordiBForgeFlow]

Some cloud providers such as Digital Ocean are blocking the SMTP ports, and advising their customers to deliver emails via API https://docs.digitalocean.com/support/why-is-smtp-blocked

We are willing to implement the API offered by Mailgun to send emails instead of SMTP.

[pedrobaeza]

It's a good option, but I wouldn't put it inside mail_tracking_mailgun, as it's goal is another and contains a lot of other things (and dependency to mail_tracking), which is huge.

I think you can develop a module mail_mailgun to do that, and recopy the needed fields. On the new version, we can converge having a base module mailgun_api, that both mail_tracking_mailgun and mail_mailgun depend on.

What do you think?