Generic GeoArrowLayer

Top-level layer for any GeoDataFrame/GeoArrow input. For now, this will be a "container widget" that stores a sequence of ScatterplotLayer, PathLayer, and PolygonLayer. In the long run, it'll pass that data into a JS GeoArrowLayer, just like any other layer. This means we should ensure the near-term API lines up with the expected long-term JS API.

There may be space for some deduplication between `viz` and the `GeoArrowLayer`.

It's also unfortunate that the name `GeoArrow` doesn't have the name recognition that `GeoJSON` does. Perhaps we should alias this object in Python to the `GeoJsonLayer` as well? Where the user passes in a GeoDataFrame or a GeoJSON object and the conversion to Arrow happens under the hood?