Implement support for union types as resource types in Azure Core

[daviwil]

This work will require:

• Update @resource decorator to validate that all union variants are model types, have equivalent key properties, segment name, and parent resource type
• Update KeysOf<T> to extract keys correctly from union type
• Add an envelope type for response type (like ResourceBody<T>) and ensure that it works correctly with existing emitters. This is intended to wrap the resource type so that traits will mix in to the envelope type and not directly in the resource type

[markcowl]

Also, I think:

• show examples of how emitters would need to be updated to process discriminated unions in our templates

[markcowl]

est: 8