SfuResponse:
| { rtpCapabilities: unknown; type: "sfu-caps" }
| { transport: unknown; type: "sfu-transport-created" }
| { transportId: string; type: "sfu-transport-connected" }
| { producerId: string; type: "sfu-produced" }
| { consumer: unknown; type: "sfu-consumed" }
| { consumerId: string; type: "sfu-consumer-resumed" }
| { message: string; type: "sfu-error" }
The reply the handler returns for a given request (shape depends on
type).