Investigate JOINs 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 other tables are joined onto rows in this table, how often this happens, how we'd get rid of those joins (if at all possible), etc. The end goal is to have a better understanding of how much application code needs changing to make this a possibility.

Confidence level for the due date: 80%