WIP: Add ability to keep personal email private

This MR adds the ability to define a private commit email address.

A generated email address (e.g <id>+<username>@noreply.gitlab.com) option is added to the user > settings > Commit email dropdown. The user can select this option in order to ensure that commit emails are kept private.

What are the relevant issue numbers?

Closes #43521 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides