Add Configuration Option for Mandatory public email

Problem to solve

Since https://gitlab.com/gitlab-org/gitlab-ce/issues/57253 we've decoupled public_email from email out of security concerns. However, some enterprise customers would like to enforce that everybody has a public email or, that everybody's corporate email is public.

Target audience

• Sidney, Systems Administrator, https://design.gitlab.com/research/personas#persona-sidney
• Sam, Security Analyst, https://design.gitlab.com/research/personas#persona-sam

Proposal

Introduce an instance level setting that, when selected by admins, uses each user's primary email address as their public email address.

Implementation plan

1. Create a migration to add a column enforce_public_email in application_settings (something like db/migrate/20250416144638_add_project_web_based_commit_signing_enabled_setting.rb)
2. Add the field to visible_attributes in app/helpers/application_settings_helper.rb
3. Add the new field to app/views/admin/application_settings/_account_and_limit.html.haml (I think this is most logical place on admin -> settings -> general -> account and limit)
4. Expose lib/api/entities/application_setting.rb
5. Allow updating via the API lib/api/settings.rb

This can (and should be) broken up into multiple merge requests- perhaps:

1. Add the database column
2. Add to the UI
3. Add to the API

Links / references

ZD https://gitlab.zendesk.com/agent/tickets/113781 (internal)