Allow `MailService.send()` to use templates from other sources in addition to file based templates.

Describe the Improvement

I want to use MailService in my extension, but I need the email messages (templates) to be editable (not to be hardcoded in a file), so when there is a change in the template I don't have to build and deploy a new image each time.



In this way I can setup a collection for email templates and use it in my extension when I need to send an email.

[Here is the implementation that I made](