support external members in roles and groups

[havetisyan]

Description

support external members in roles and groups #3255

the go client requires changes to switch to PrincipalName syntax to support the new external members.

Contribution Checklist:

• The pull request does not introduce any breaking changes
• I have read the contribution guidelines.
• Create an issue and link to the pull request.

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[havetisyan]

/gemini review

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Code Review

This pull request introduces support for external members across Athenz components, including schema updates, client/server modifications, and new validation logic. The changes involve adding new types like ExternalMemberName and PrincipalName, updating access check logic, and implementing a validator for external email members. Feedback suggests improving error handling in ZMSImpl.java by returning a 400 Bad Request instead of a 401 Unauthorized when an invalid principal name is provided, ensuring consistency with other parts of the codebase.