Add ability for papermill to integrate into bookstore

[MSeal]

Ideally a user should be able to run papermill and set the output path to a bookstore reference. The papermill run would generate a new version of the notebook and save against the new version path.

A few ideas for how to approach:

1. Add an (or use an existing) endpoint to reserve a new version and return the path to that version. Papermill could then save to that final location as it executes. This has the downside of generating an empty version temporarily in bookstore by populating the notebook as cells are computed. This might need to be accounted for in bookstore primitives?
2. Make the config loader from bookstore importable in other projects. Papermill could load this config and reuse the logic bookstore uses. This has the downside that the library is not an API contract that might be out of sync with bookstore version, meaning we'll need versioning / compatibility between versions.

There's probably other approaches but I think 1) could be done without major constraints of changes made. Welcome to thoughts on the topic. It's not a high urgency item.