Blueprints: Add support for OS phase 2 images

[jgallagher]

#8538 adds a HostPhase2DesiredContents to OmicronSledConfig, and the subsequent few PRs implement the sled-agent side to act on it.

This issue tracks adding a corresponding field to BlueprintSledConfig (and the associated database work to store it).

[davepacheco]

I believe this was done in #8570.