Generic GeoArrowLayer

[kylebarron]

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?

[kylebarron]

cc @ncclementi ref #632

[kylebarron]

Adding this to the next minor release milestone because I think it's important for a user to be able to pass data returned from geoarrow.rust.io.read_* directly into Lonboard. That means implementing the new generic GeometryArray type defined in geoarrow/geoarrow#43