Describe JsonPatch correctly in ApiExplorer

[Eilon]

In the JsonFormatter package we need to register an IApiDescriptionProvider that mutates parameters of type JsonPatchDocument<T> into being Operation[].

See domaindrivendev/Swashbuckle.AspNetCore#199 for what some current workarounds are. It would be nice if we had this built-in to avoid the need for workarounds.

[jbagga]

@rynowak Any thoughts? Should the IApiDescriptionProvider be in the ApiExplorer package or Formatters.Json? The patch needs JsonPatchDocument to be applied, so it should be mutated to Operation[] after that?

[rynowak]

We should consider moving the IApiDescription and the stuff needed to implement one to abstractions. We should however keep the infrastructure needed to read descriptions in core.