Investigate constraints for a vertically sharded ci_builds table

As part of &5883 (closed) we want to look into what it takes to vertically shard the ci_builds table.

In this issue specifically we should look into how the constraints applied to this table (e.g. foreign keys), how they would work if ci_builds were to reside in its own database, how many tables we'd have to move over to keep the constraints, etc.

Confidence level: 80%