Add support for MTA bus data

[wilsonchaney]

It might be worth it to add support for MTA bus data. It's a separate API from the realtime subway one, but has lots of similarities.

We could probably use their SIRI API (which supports JSON responses) and just parse the responses into pydantic models, similar to what you did for subway data.

I haven't looked into this in much depth but I think it wouldn't be too hard. I might take a stab at it if I have time in the coming weeks, figured I'd file an issue to get your thoughts.