Geo: Implement selective sync support for the various FDW queries

Follow up after ~"product discovery" in https://gitlab.com/gitlab-org/gitlab-ee/issues/4645:

Implement selective sync support for the various FDW queries in https://gitlab.com/gitlab-org/gitlab-ee/tree/master/ee/app/finders/geo

• 1. Implement the FDW queries for selective sync under a feature flag - %11.9 - %11.10
• 2. Test the performance of these queries with a large dataset - %11.10
• 3. Based on the test results make FDW a hard requirement or implement https://gitlab.com/gitlab-org/gitlab-ee/issues/7109#note_136712102 - %12.0
• 4. Remove the feature flag and the legacy queries - %12.1