feat: open-api default response for dart

[midzelis]

This PR allows the nestjs controllers to be annotated with the status code specific responses, like @ApiCreatedResponse and @ApiOkResponse - and allows the use of different return types for each response. This works perfectly fine with the typescript client, however, the dart2 open-api generator will pick the type of the first 2xx response as the return type of the the operation - which is not correct if there are divergent types for each response code. To allow the the use of typescript to make use of the additional type information, and also support Dart as best we can - you can make use of the @ApiResponse{status: 'default', ... } annotation - this PR patches the api.mustasch dart template to use the default response's return type instead of the first 2xx response. The coding guidelines would be - if you are planning to different types for different response codes, also include a default response with a type that is the logical union of all the properties each type has. This doesn't give you as nice a type in the the response, but its the best that can be done for the Dart language, as it does not support union return types like typescript and other languages can.

Note - the Dart api generator will generate Dart methods for each operation - one that is typed, and one that uses a generic Response object that has the format <operation>WithHttpInfo. A dart client would ALWAYS be able to use the generically typed form of this method.

This has been split from: #8480

[jrasm91]

Is it worth submitting a PR upstream for this functionality?

[midzelis]

Not sure - the result is not perfect (you still need to define a default response), but it might be better than how it is today. This was inspired by this change: OpenAPITools/openapi-generator#128 I'll submit a PR upstream a bit later and see if they want to merge it.

[zackpollard]

My opinion is that lets see if they're willing to accept this upstream, and then if not we can consider whether we want to add this just for our solution. Imo this leaves us in a weird spot where we can use the annotations but only have partial support, I feel like we may just be better not using those annotations for both platforms.