Support Multiple Upstream WebSocket Connections for HA Setups

[angel-ding-cb]

This PR enhances the WebSocket proxy to support connecting to multiple upstream servers simultaneously. This is particularly useful for high-availability setups where multiple backend servers may be available but only one is actively producing data.

Changes

• Modified Args to accept a vector of URIs via a comma-separated list
• Enhanced WebsocketSubscriber to track and log connection status for each upstream
• Added metrics to monitor multiple upstream connections
• Updated main.rs to spawn a subscriber task for each upstream URI

Notes

• Only the active upstream will send data, avoiding duplication issues

[danyalprout]

@angel-ding-cb looks good overall

Only the active upstream will send data, avoiding duplication issues

Can you elaborate on how this will work with rollup-boost and op-rbuidler?

Also just an FYI looks like you may need to run cargo fmt --all for the CI to pass. Can we also add a test to validate multiple WebsocketSubscribers and a single listener