Allow caching arbitrarily named artifacts per platform config

It would be nice to have a way to upload an arbitrarily named artifact in a qtbase work item, and then be able to download it in a qtdeclarative work item, which is cached per platform config key, and also probably needs to be keyed somehow by qtbase work item.

This would potentially allow building qdoc in a qtbase integration, and download further it down in other work items.