Add endpoint for accessing the shas of unrolled PR builds

[rylev]

We now have support for unrolling a rollup and building artifacts for them. The shas identifying these artifacts are then posted to GitHub, but they are not stored anywhere else. It would be nice to expose and endpoint that allows others to ask for this information.

We could do this by adding a database table which stores the mapping of PR to sha. However, since creating the sha should be reproducible, it might be possible to do this on demand.

One additional note, since these builds have proved useful for more than just perf (e.g., for catching regressions), it feels a bit weird that this lives in rustc-perf. I'm not sure where else this should go though.

[Mark-Simulacrum]

We could move the code to triagebot and keep the database storage there too, wouldn't be too hard.

FWIW I don't think we should rely on being able to reproduce the merges, too annoying if we change the commit message or GitHub changes the raw commit formatting (e.g., different timestamps).