Add support for multiple user emails

[fdobrovolny]

The code now fetches a list of user emails instead of a single email address per user. The user structure and corresponding parts of the code have been updated to handle and check multiple email addresses. Additionally, a helper function is added to check for the existence of an email in an email list.

[alexandear]

Thanks for the PR. I'll review and merge it later today or tomorrow.

Please ensure, the tests are green.