Allow multiple stores in Dataverse

[qqmyers]

For use cases in which it makes sense for data files to be stored in more than one location (such as keeping sensitive data local, providing different types of stores for different data sizes, enabling multiple stakeholders to contribute storage behind a Dataverse instance, allowing new files to go to s3 without moving existing files from a local file store), it would be useful for Dataverse to support configurations with more than one store enabled.
I'm preparing a PR based on work for TDL that provides such support. It implements the core capability for Dataverse to reference files in multiple locations, supports backward-compatibility (requires some config changes but no database changes or need to move files), and provides a mechanism to specify a default store and to associate a store with a Dataverse (so that all datasets within that Dataverse use the specified store.).