[FEATURE] Support STARTTLS command

[rcbop]

New feature request checklist

• [x] I have updated go-smtp-mock to the latest version
• [x] I have read the Contribution Guidelines
• [x] I have read the documentation
• [x] I have searched for existing GitHub issues

Feature description

Hi, thank you for providing this great tool for testing. It would be amazing if the mock server would support STARTTLS command.

[bestwebua]

@rcbop Thanks! New PRs are welcome. Otherwise I'll consider implemenatation of this feature next few months.

[oshkiole]

Hi @bestwebua , this tool is really great and useful! Are you still planning to implement STARTTLS command?

[bestwebua]

Hi @bestwebua , this tool is really great and useful! Are you still planning to implement STARTTLS command?

@oshkiole Hello, Oleksandr! Yup. You're welcome to this topic disscussion and PR. Feel free to ask any questions.