Implement Functionality in send() Methods of Mailer Classes

[Risad212]

Currently, the send() methods in SmtpMailer and SendGridMailer are empty, making it hard to see how each mailer works in the example.

Suggested Fix
Add a simple implementation inside each send() method to make it clear which mailer is being used.
Update the SendWelcomeMessage class to include a method, like sendWelcome(), that calls send() on the mailer.
This will help others understand and test the Dependency Inversion Principle in action with this example.