Use benchmark run roles to identify a Quent asset

[TomAugspurger]

The (nvidia-only) quent-open db <run-id> feature currently has to infer which assets in a benchmark run is a quent asset. To avoid that, we could attach roles to a benchmark run:

"roles": [{"type": "quent", "filename": "asset-filename.zip"}]

Given a benchmark run with that role, quent-open could automatically use that asset without having to infer.

Assets also include media types. Perhaps we could standardize on an unofficial media type for quent and always use those.