[CT-1611] Create protobuf definitions to match node objects

[gshank]

Some of this work was done on a different ticket (#5863), but things have gotten confused, so opening a fresh ticket.

This is to create protobuf definitions and code to generate protobuf derived objects from the various nodes, such as ModelNode, SeedNode, AnalysisNode, etc. The goal is to eventually generate our artifacts from these definitions and include some of these in structured logging. The protobuf definitions would eventually take the place of our jsonschemas, though we could have both for a while.

[github-actions]

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