Email templating engine

[stefanotroncaro]

Create templates submodule

Features

• jinja2 based rendering
• mjml rendering via mrml library
• inheritable BaseTemplate and BaseEmailTemplate pydantic schemas to relate template files and required arguments
• flexible post rendering pipeline configurable on 3 levels: template definition, render method call, and service instance
• component based template definition example, defining a theme to follow a design system for template consistency
• module usage examples

Proof of testing

Sample email

Test suite

[DanTcheche]

I'm not sure about the template not being inyectable here. Is there a specific reason?
I'm thinking about maybe A/B testing where you want to send 2 different new user emails to users depending on X factor.

[stefanotroncaro]

I totally agree with you.

I want to add that functionality on a later refactor, to clean up the email service so that it is closed for modification (which it currently is not, as it basically requires a new method for each email type).

I didn't do it here because I didn't want to scope creep this PR. Would you be ok with working on that on a different PR?