Geo: Migrate CI artifacts into their own registry table

The ci_job_artifacts table is growing quickly, and it's becoming slow to do queries against the file_registry, even with indexes in the mix.

This split allows us to make simpler queries and add optimizations, such as querying only new records after a certain ID.

Part of #5039 (closed)

To do:

• Add migration specs
• Add Geo migration CI stage: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5090