Add SMTP email service integration using Spring Boot Mail Starter

[faizakram]

This pull request introduces SMTP email service integration to the application using the spring-boot-starter-mail dependency, along with a new test endpoint to verify email functionality.

Key Features

1. SMTP Email Integration:

   • Configures the application to use an SMTP server for sending emails.
   • Supports sending simple text emails, HTML emails, and emails with attachments.

2. Test Endpoint for Sending Emails:

   • Adds a POST endpoint to send emails using SMTP with dynamic parameters for the recipient (to), subject (subject), and body (body).
   • Facilitates quick testing of email functionality to ensure proper SMTP configuration.

3. Flexible Configuration:

   • Uses Spring Boot’s application.properties or application.yml for configuring SMTP settings (e.g., server host, port, username, password, etc.).
   • Allows dynamic configuration to adapt to different environments (development, testing, production).

4. Ready for Future Enhancements:

   • Provides a foundation for implementing advanced email features, such as templated emails, batch email processing, and event-driven notifications.

How to Configure SMTP Email Service

To configure the SMTP email service, add the following properties to your application.properties or application.yml:

spring.mail.host=smtp.example.com
spring.mail.port=587
spring.mail.username=your-username
spring.mail.password=your-password
spring.mail.properties.mail.smtp.auth=true
spring.mail.properties.mail.smtp.starttls.enable=true

### How to Test

1. **Test Endpoint for Sending Emails:**
   - Adds a `POST` endpoint to send emails using SMTP with dynamic parameters for the recipient (`to`), subject (`subject`), and body (`body`).
   - Useful for developers and testers to validate the email service integration quickly.

2. **Dynamic Email Content:**
   - Supports dynamic email content by allowing `to`, `subject`, and `body` parameters to be passed in the request URL.
   - Enables easy testing with different email content variations.

### How to Test

To test the SMTP email service, use the following `POST` request:

```bash
curl --location --request POST 'http://localhost:8080/email?to={{$placeholder}}&subject={{$random.alphanumeric(8)}}&body={{$random.alphanumeric(8)}}'