websockets to return latest data (as opposed to every frame)

[beasteers]

I already made an initial effort on this, but it could use a second opinion and some testing to know if this is the best way to be doing this.

https://github.com/VIDA-NYU/ptg-api-server/blob/main/app/core/streams.py#L109-L176

Additionally, here's where it's used. I'm not totally convinced about this line latest = last_entry_id is None or '$' in last_entry_id so maybe it should be overridable by query parameter? idk

https://github.com/VIDA-NYU/ptg-api-server/blob/main/app/routers/data.py#L141-L153