Use public email for seat usage group members

What does this MR do?

As discussed in #292086 (closed) and the related issues, the logic behind whether or not to display a group member's email address in the seat usage page needs to change.

Previously it would use Group Managed Accounts to determine if it should or not, but now we are going to fall back to a more basic approach of using the public email address.

This MR addresses that by exposing the user's public_email (if present) - if it's not present, we display the private email message and new tooltip (see below).

The frontend Vue component checks if the email value is present or not to determine if it should be displayed, so no logic changes need to take place there (source).

Testing

In order to test this change, you'll likely need to do the following:

• Enable the feature flag: Feature.enable(:api_billable_member_list)
• Navigate to the seat usage view: /groups/your-group/-/seat_usage

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.