Move mailing functionality into a submodule (Architecture)

@Maintainers: Can I help here? Could you please eventually give some feedback on your plans and if you agree?

Hi @anybody, the transactional email sending feature was the first main milestone of the module.

If you don't want to send transactional emails, enabling the module will not force you to. You still have to configure MailSystem/Symfony mailer to do so. I understand your idea of separating features into submodules, and I'll look into it when I'll implement some more features.

Thank you @renrhaf! That sounds great!

Yes indeed it would be helpful to have the email functionality split off like described and a minimal base module. That would make it easier for us to implement mail-unrelated functionality and the focus of each submodule more clear. And you won't have to mess around with features you don't need :)

I think it would indeed make sense to be either done by you or in a time when other development pauses to not lose precious time with conflicts.

We're absolutely willing to help with the future of this module. Thank you!

PS: Having that done will also unlock ✨ Brevo newsletter (un)subscribe API Active with its postponed issues. :)