AsyncAPI support

[torikki-tou]

I am using grpc-gateway together with grpc-websocket-proxy to expose gRPC streaming endpoints as WebSocket connections.

When generating an OpenAPI specification from my proto files, there is naturally no information about these WebSocket endpoints, since OpenAPI does not support WebSockets (and likely never will).

I was wondering whether it would make sense to support generating an AsyncAPI specification alongside OpenAPI, so streaming / WebSocket-based endpoints could be documented as well.

I would also be willing to work on this feature in the coming months (time permitting). That said, I haven’t researched the topic in depth yet and may be missing fundamental limitations or design constraints that would make this impractical.