Resolves #2034

Initial draft PR opened for conversation:

Current implementation:

1. Adding a new observer actor that handles external connections (unauthenticated)
2. Observer sits at simplex layer instead of p2p layer, since we're not a participant in the p2p network
3. broadcast certificates in obserer
4. broadcasts replay certificates

Things that need alignment on:

1. Seems to me that just open a websocket port for observers to listen might be the better solution for this case (if people just want to observe the network consensus progress), Though current implementation takes in a Listener type currently, ideally we take in just a WebSocketStream, let me know if there might be other scenarios that require lower level Listener
2. imo we should by default not broadcast replay messages (tons of waste basically), but in case that listener wants historical state (for backfill or whatever), it could be helpful. Do we want to make it configurable or just simply not broadcast them?

Remaining work:

1. update tests & examples (reshare / bridge, etc) with new setup
2. tests
3. example client or websocket client (depending on if we use Listener or WebSocketStream)
4. docs