fix add mailable to newsletter subcription

[abdielbytes]

​

Description

I added mails to newsletter subcriptions. Users will now receive a welcome message when they subcriibe to our newsletter ​

Related Issue (Link to Github issue)

https://github.com/hngprojects/hng_boilerplate_nestjs/issues/820 ​

Motivation and Context

​

How Has This Been Tested?

This change has been tested on postman and through unti testing. ​

Screenshots (if appropriate - Postman, etc):

​

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change) ​

  Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.