Describe the feature

We want to connect Replica nodes to the sequencer (or generally an "upstream" node) via WebSocket to subscribe to a few events. We can add a new CLI argument for the WebSocket endpoint, but also wonder if we could promote the existing sequencer-http to support both HTTP and WebSocket. Existing users can keep using HTTP, but we (and hopefully more) can put in WS for both the "built-in" transaction forwarding and more custom use cases like event subscription.