datasets not getting propagated into initial blueprint from RSS

[smklein]

The initial RSS-constructed blueprint is currently being created with an empty set of datasets.

This is somewhat okay, because this scenario mimics an existing system which upgrades across the #6229 boundary -- it'll have a blueprint without the datasets field, and then will suddenly have no datasets.

However, it's information that we have, and should back-fill. Without auto-populating the blueprint, it'll be harder to clean up some of the backwards compatible cruft, such as checking CockroachDB for dataset UUIDs.

This issue tracks such population.