Add a basemap to geospatial Superset plots

[MattTriano]

Mapbox is the default basemap provider for Superset's deck.gl geospatial plots. I had been reluctant to use Mapbox as it is a commercial product and I only want some functionality that I know is freely available (e.g., openstreetmaps, or other providers indicated in leaflet tutorials).

After spending some time searching for existing functionality for using a non-Mapbox provider and reading through this aging Superset Improvement Proposal, calling Mapbox the "default" might be a bit aspirational, as it seems to be the only currently supported option.

Fortunately, Mapbox appears to have a free tier. I don't want to add in a dependency that would require a user to enter a credit card number into a site (or otherwise have to configure any kind of potential payment scheme), but if I can create an account without having to set up a payment profile, then Mapbox is fine for now.

• Determine if I can get a Mapbox API key without setting up a payment profile,
• If "yes" for the above, add a MAPBOX_API_KEY environment variable to the startup process and add notes to the Superset setup doc.

[MattTriano]

I made a Mapbox account 6 years ago to play around with the product back then. I hadn't set up a payment then (I checked the payments interface and there weren't any configured payment profiles). I made a new API token, put it into my .env.superset file using the env-var name MAPBOX_API_KEY, docker compose down -> docker compose up, and checked a dashboard that had a geospatial plot, and it had a basemap. Nice and easy.

The terms and conditions had warnings that the owner of the API token is responsible for any incurred charges, even if someone steals their token and is incurring the charges elsewhere, so I'll switch away from Mapbox if it becomes easy.