feat: support looking up github user by username

[jaredfholgate]

Overview/Summary

Allow searching for GitHub users by username or email.

This PR fixes/adds/changes/removes

1. Feature Request: allow searching for github users by username as well as email Azure-Landing-Zones#2398

Breaking Changes

None

Testing Evidence

Please provide any testing evidence to show that your Pull Request works/fixes as described and planned (include screenshots, if appropriate).

As part of this Pull Request I have

• Checked for duplicate Pull Requests
• Associated it with relevant issues, for tracking and closure.
• Ensured my code/branch is up-to-date with the latest changes in the main branch
• Performed testing and provided evidence.
• Updated relevant and associated documentation.

[Copilot]

Pull Request Overview

This PR enables the support for looking up GitHub users by username in addition to email, in order to provide more flexible approver identification.

• Splits the input approvers into email-based and username-based lists.
• Adjusts user selection and error messaging accordingly.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
modules/github/team.tf	Updated logic to separate and validate approvers by email and login.
alz/github/variables.tf	Updated variable description to reflect support for usernames.