Avoid use of any/interface{}

Overview

Most fields in the sdk are represented using concrete types, either primitives or structs derived from the openapi spec. However, we occasionally fall back to any or interface{} types. This happens when we haven't handled an edge case in the spec, or when we represent a complex oneOf type based on a rust enum. I think this task has a few parts:

• Fix edge cases in the generator so that we don't fall back to any for unknown types.
• Decide how to represent oneOf types in a relatively ergonomic way.
• Update the generator so that returning an any/interface{} is an error.

Implementation details

No response

Anything else you would like to add?

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