Spring Boot: Email service layer

[preetkaran20]

All Java, no Docker. Touches VulnerableApp's Spring Boot source only.

Add spring-boot-starter-mail to build.gradle
Add spring.mail.* and vulnerableapp.email.* properties to application.properties with env-var overrides
Implement EmailService, EmailServiceImpl
Add EmailTestController with /email/test endpoint
Unit tests for EmailServiceImpl

[preetkaran20]

Depends on #650

[preetkaran20]

#648 parent issue

https://github.com/SasanLabs/VulnerableApp/wiki/Design-Document:-SMTP-Server-Integration-for-VulnerableApp Design doc