feat: Updated email templates with user information

[pmagictech]

Description
This pull request enhances the existing email templates by including the username of the recipient. This update aims to improve the personalization of the emails, making the communication more engaging and user-friendly. By making the User Entity object accessible in the email templates, Shield users can now easily add user-specific details, tailoring the emails to better suit their needs.

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

[kenjis]

Thank you for sending this PR.

We don't accept PRs without test code that proves the change works properly.
Please add test code.
See https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/pull_request.md#unit-testing

[datamweb]

Thanks for PR.
You can get help from Email Test for write the test.

[datamweb]

Can you update description https://github.com/codeigniter4/shield/blob/develop/docs/customization/views.md#customize-content and explain that user data(User Entity object) in email tpls is now available?

[datamweb]

LGTM🫡!

[kenjis]

@pmagictech Thank you!

[pmagictech]

@kenjis You are welcome