codegen: Union not fully supported for REST-XML protocol.

[jasdel]

The SDK's code generation does not currently support REST-XML protocols

• https://awslabs.github.io/smithy/1.0/spec/aws/aws-restxml-protocol.html?xml-shape-serialization

Example of Unions being used with Amazon S3 API
https://github.com/aws/aws-sdk-js-v3/blob/e6b2ae1efab395cf645a8e280a4e21dd77b5ca12/codegen/sdk-codegen/aws-models/s3.2006-03-01.json#L582-L607

[github-actions]

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