Allow group Owners to view enterprise users emails on UI/Members page

What does this MR do and why?

Related to #423493 (closed)

This MR is similar to Members API: Allow group Owners to obtain enter... (!135873 - merged), but for UI(Group members and Project members pages).

!141780 (merged) is related frontend change.

In this MR we update MemberUserEntity to expose user's email attribute if current user is

• GitLab instance Admin with turned on Admin Mode
• group Owner of the group with the paid subscription and the user is enterprise user of that group

MemberUserEntity is only used for

• Project members page: https://gitlab.com/gitlab-org/gitlab/-/blob/4cb59e1d8ff3231fa1cb27691a9ddeae27f61f68/app/views/projects/project_members/index.html.haml#L40
• Group members page: https://gitlab.com/gitlab-org/gitlab/-/blob/4cb59e1d8ff3231fa1cb27691a9ddeae27f61f68/app/views/groups/group_members/index.html.haml#L24

to pass members data in frontend/Vue.js context via data- HTML attribute.

This MR and the frontend MR are independent and could be merged in any order.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Go to /groups/flightjs/-/group_members
2. View Page Source and check the emails.