Geo: Replicate Pipeline Artifacts

Summary

A new data type, Ci::PipelineArtifact was introduced in GitLab 13.3.

It would be useful for this to be replicated to a secondary (#235918 (comment 395623970)).

Proposal

Enlist help from grouptesting to implement replication for PipelineArtifact using the self-service framework with assistance from Geo. This is what the SSF was intended for and presents another opportunity to receive feedback on and improve the SSF.