This introduces a new feature to the ArkService API that allows clients to subscribe to payment events. Here's a breakdown of the changes:

1. **OpenAPI Specification (`service.swagger.json`):**
   - A new endpoint `/v1/payments` is added to the API, supporting a `GET` operation for streaming payment events.
   - New definitions `v1GetPaymentsStreamResponse`, `v1RoundPayment`, and `v1AsyncPayment` are added to describe the structure of the streaming responses.

2. **Protobuf Definition (`service.proto`):**
   - Added a new RPC method `GetPaymentsStream` that streams `GetPaymentsStreamResponse` messages.
   - Defined new message types: `GetPaymentsStreamRequest`, `GetPaymentsStreamResponse`, `RoundPayment`, and `AsyncPayment`.

3. **Generated Protobuf Code (`service.pb.go`, `service.pb.gw.go`, `service_grpc.pb.go`):**
   - The generated code is updated to include the new RPC method and message types.
   - The gateway code includes functions to handle HTTP requests and responses for the new streaming endpoint.

4. **Application Logic (`covenant.go`, `covenantless.go`):**
   - New payment events channels are introduced (`paymentEventsCh`).
   - Payment events are propagated to these channels when a round is finalized or an async payment is completed.
   - New event types `RoundPaymentEvent` and `AsyncPaymentEvent` are defined, implementing a `PaymentEvent` interface.

5. **gRPC Handlers (`arkservice.go`):**
   - Added logic to handle `GetPaymentsStream` requests and manage payment listeners.
   - A new goroutine is started to listen to payment events and forward them to active listeners.

Overall, this patch extends the ArkService to support real-time streaming of payment events, allowing clients to receive updates on both round payments and async payments as they occur.