Geo: Implement selective sync support for the attachments FDW queries
Follow up after ~"product discovery" in https://gitlab.com/gitlab-org/gitlab-ee/issues/4645:
Implement selective sync support for the FDW queries in https://gitlab.com/gitlab-org/gitlab-ee/tree/master/ee/app/finders/geo/attachment_registry_finder.rb
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1. Implement the FDW queries for selective sync under a feature flag -
2. Test the performance of these queries with a large dataset