Fix codegen for unions with the `@httpPayload` trait (#2969)

This PR incorporates the new test cases in Smithy from
https://github.com/smithy-lang/smithy/pull/1908, and adds support to
`@restXml` and `@restJson1` for unions with the `@httpPayload` trait.
This resolves https://github.com/awslabs/smithy-rs/issues/1896.

This also fixes code generation for the latest `medicalimaging` SDK
model.

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