S3/GCS realization uploading

[zombiezen]

#43 covers using realizations from remote sources, but for a full user experience, we should have a built-in way of having the store server upload its build artifacts to a bucket. This feature...

• should not block the build from proceeding
• should not block the user from receiving build results
• should block the store server from shutting down
• must verify the integrity of the files it is sending to the bucket. If the integrity check fails, then the object will be deleted.
• should be as atomic as possible. If an error occurs, then orphan objects should not be left behind.
• must not re-upload the same objects (to save on bandwidth).
• may support some basic path rewriting from discovery documents, but some
• must allow interactive invocation outside of the store server
  • copy store object (and dependencies) from local store to bucket
  • copy realization and signatures (also for referenced build inputs) to bucket
• should allow configuring multiple destinations
• may be resumable on server exit. Not sure what happens if we reconfigure between runs.

Backends to support

• Amazon (S3)
• Google (GCS)
• S3-compatible, custom endpoint (Minio, etc.)

In the future, we could consider supporting HTTP/WebDAV servers with PUT.