#12 Implement mail service

[IlyaLisov]

close #12

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PR-Codex overview

This PR focuses on adding email functionality to the task list app.

Detailed summary

• Added MailType enum to represent different types of emails.
• Created Reminder interface and ReminderImpl class for sending reminder emails.
• Added MailService interface and MailServiceImpl class for sending emails.
• Added MailProperties class to configure email properties.
• Added MailConfig class to configure the mail sender.
• Updated UserService implementation to send registration emails.
• Added email templates for registration and reminder emails.
• Updated TaskService and UserRepository to get task author information.
• Added ReminderImplTest and UserServiceImplTest for testing the new functionality.

The following files were skipped due to too many changes: src/test/java/com/example/tasklist/service/impl/AuthServiceImplTest.java, src/test/java/com/example/tasklist/service/impl/TaskServiceImplTest.java, src/main/java/com/example/tasklist/service/impl/MailServiceImpl.java, src/test/java/com/example/tasklist/service/impl/MailServiceImplTest.java, src/test/java/com/example/tasklist/config/TestConfig.java, src/test/java/com/example/tasklist/service/impl/UserServiceImplTest.java

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