feat(newsletters): add subscribe endpoint

[Solomonuche]

This PR contains the implementation of [POST] /api/v1/newsletters [UNPROTECTED Endpoint]

Description

Develop a backend endpoint to handle requests for newsletter subscriptions. The endpoint will accept an email address, validate and sanitize it, and then handle both successful and failed validations appropriately. If the email passes validation, it will be stored in the database and a 'success' response will be sent to the client. If validation fails, an error message will be returned.

Related Issue (Link to issue ticket)

This is an implementation of an open issue Link

How Has This Been Tested?

This implementation has been thoroughly tested with fastapi openapi interface and postman

Screenshots (if appropriate - Postman, etc):​

Successful response

Error response

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.

[joboy-dev]

@bolexs this is okay

[SundayMba]

@bolexs okay

[zxenonx]

Thanks @Solomonuche. Good.