Improve email service by spliting the providers into different classes

[luizfonseca]

First, just wanted to say that I really like the project and the work you've done so far!

This PR is mostly a minor improvement on how we are capturing the email provider information -- main reason because i wanted to add https://resend.com/ but didn't want to make the if block larger than it is.

This PR:

• Creates an abstract class that mailers can extend. Main reason to do it this way is to ensure consistent typings/returns and hide those details on each mailer that implements it
• Creates a mailer class for each, that way we can isolate any future nuances and eventually add more providers (e.g. resend, in another PR) 👌🏽

Feel free to discard or take over the PR if you don't like the approach of classes, I'm not married to it.

[byn9826]

Thanks for your PR, looks great to me. Very solid improvement.