Support Choropleth's locationmode in Choroplethmapbox #5048

A choropleth trace supports specifying locations as high-level string names/IDs, via the locations and locationmode keys. This is very convenient and makes it easy to get a nice plot 👍

A choroplethmapbox trace seems to not support this, and thus requires specifying any geometries as a manual GeoJSON file. It would be nice if choroplethmapbox supported the same automatic "inference" of appropriate geometries too.

Related issues:

• Introducing choroplethmapbox traces #3988 mentions the possibility of this, but it seems there's no progress:

  locationmode. The current behavior would correspond to locationmode: 'geojson-id'. Some users might expect to identify GeoJSON features using their properties.name (this would correspond to e.g. locationmode: 'geojson-prop-name'. Moreover, we could also support the locationmode values found in geo traces: 'ISO-3', 'USA-states', 'country names'

• Add new choroplethmapbox locationmode for geojson properties.id #4154 was, I think, about generalising the GeoJSON support in choroplethmapbox, but not adding the no-GeoJSON mode as here

• Add support for custom geojson polygons in choropleth traces #4267 was the opposite, I think: adding support for GeoJSON to choropleth