Support multiple outputs in a proto node definition with return struct unpacking in the node macro

The node macro should be updated to convert a returned struct (with each field annotated with an attribute for metadata like the output name) into the necessary node definitions to unpack the struct as separate outputs. This could occur by generating a subgraph, but an approach which allows for avoiding that would be preferred.

Start by locating the node macro and reading how it currently generates node definitions and handles return values. Compare the existing generated structure with the desired annotated return struct behavior, and determine whether direct output unpacking or a generated subgraph fits the macro design. Done means a returned annotated struct produces separate named outputs without breaking existing node definitions.

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