Add @oneOf decorator to JSON Schema emitter

[adamscybot]

Clear and concise description of the problem

I am using the JSON Schema emitter, as opposed to the OpenAPI emitter.

In the JSON Schema emitter, unions by default output as anyOf, which I believe to be sensible, and it is also consistent with the behaviour of the OpenAPI emitter.

However, unlike the OpenAPI emitter, there appears to be no parallel to the @oneOf decorator to force the use of oneOf over anyOf.

I would quite like this capability for some use cases in my schemas. Side point, but any workaround that I could do in the short term would be awesome too!

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