Make emailService.js reusable and make different email templates

[StefanPenchev05]

The current implementation of the emailService.js file is specifically tailored for sending verification emails during user registration. This limits the reusability of the code for other types of emails that the application might need to send, such as password reset emails, welcome emails, or promotional emails.

The goal of this issue is to refactor the sendVerifyMail to sendMail function in emailService.js to make it more generic and reusable. This involves abstracting the email sending logic and allowing different email templates to be used based on the type of email being sent.

Additionally, we should create a set of email templates for different scenarios. These templates should be customizable with variables for things like the recipient's name, verification links, etc.

By making these changes, we can ensure that our email service is flexible and can handle a variety of email sending tasks.