Add OpenAPI docs and UI for our APIs

[zweizeichen]

Now that we have switched to gRPC for our cluster's internal communication we can generate OpenAPI docs by using grpc-gateway on which jumpgate is based. The individual service's Protobuf annotations are there, we just need to combine the individual generated Swagger docs using something like swagger-combine and package them up for deployment with swagger-ui. We may even be able to use parts of esi-swagger-ui so our docs would look like ESI's documentation.