Add decorator to allow specifying a common (error) response schema for a HTTP service

[johanste]

Clear and concise description of the problem

It is very common (and a best practice) to have a common basic error schema in HTTP servers. Many web frameworks allow you to provide global error handlers etc. that enforces this. OpenAPI moonwalk has added the apiResponses in the initial proposal that captures this.

The current pattern of having operation templates that blast in the common error schema for each operation in the service adds significant complexity to the templates - and it forces emitters that want to have a common definition to reverse engineer/normalize the common aspects. It is much easier to expand than it is to collect when emitting.

This also extends to common headers that apply for each operation. Which we currently have tried to solve with service traits - which, again have introduced significant complexity in our templates in azure libraries.

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