feat: add email/smtp plain auth method

[ip-qi]

• Added additional SMTP authentication method 'PLAIN' which sends single command for user authentication to the mail server when sending emails.
• Added new config parameter to support change in SMTP AUTH method:
  $SMTPAuthMethod = 'LOGIN' <- which is by default set to LOGIN to avoid breaking existing email setups.

This can be overriden within Config\Email to utilize 'PLAIN' AUTH method

NOTE: 'LOGIN' method could not be tested currently from my part so if possible someone test it please, i have manually tested 'PLAIN' authentication method and it is working as expected.

Explain what you have changed, and why.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[mergeable]

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[michalsn]

We should add this new option to the app\Config\Email.php file too.

We also need a changelog entry - the file will be available after merging #9464, so you will have to rebase.

[michalsn]

Thank you for the updates.

The changelog is now merged, so you can rebase and add an entry there (https://github.com/codeigniter4/CodeIgniter4/blob/4.7/user_guide_src/source/changelogs/v4.7.0.rst), under the section: "Enhancements -> Libraries". You can check previous changelogs to see what convention we use.

[michalsn]

@ip-qi Thank you and congratulations on your first contribution to the project!

[ip-qi]

@ip-qi Thank you and congratulations on your first contribution to the project!

@michalsn You are most welcome! I really appreciate the help, collaboration, and direction from both you and @paulbalandan.