Add organization selective sync option to Geo sites

What does this MR do and why?

Add ability for .com sysadmin to specify a set of organizations to replicate to a secondary site.

• Frontend for: Geo Backend - Add "selective sync by organizati... (#514251 - closed)
• Next step: Geo Frontend - Support search filter on "select... (#571426 - closed)

References

Closes #570585 (closed)

Screenshots or screen recordings

Before	After

Demo

Screen_Recording_2025-09-22_at_6.24.14_PM

How to set up and validate locally

1. Enable Geo on your GDK: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/geo.md?ref_type=heads.
2. Ensure your instance has EE license.
3. Enable geo_selective_sync_by_organizations feature flag in /rails/features.
4. Go to /admin/geo/sites and edit a secondary site.
5. Test out the Selective synchronization dropdown.

MR acceptance checklist

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