Implement support for plain bundle metadata

[exdx]

Based off #178, there is an exploratory design for a minimal set of metadata plain bundles can expose. This issue involves implementing the handling of the metadata on the provisioner side and updating the plain bundle spec to describe the metadata supported in plain+v0 bundles. See #172 for an example implementation.

A/C:

• plain provisioner knows how to parse and interact with metadata provided in a plain bundle
• plain bundle spec is updated to describe the metadata format associated with plain bundles

[exdx]

See https://hackmd.io/Tk6uXGZ2SKqreteOUAF-NA for the design doc

[github-actions[bot]]

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[tylerslaton]

Working on this is on the roadmap but we need to find the time to do it. The documents that @exdx linked should guide us forward here, at least as a starting point.