Support additional Geocoding providers

We should add additional providers for Geocoding data.

Some possibilities:
- [Mapbox](https://docs.mapbox.com/api/search/)
- [Geocod.io](https://Geocod.io) (US / Canada only 😢)
- [locationiq.com](https://locationiq.com) (US / Canada only 😢)

I think to start off with, Mapbox makes the most sense as an _additional_ provider. We'd still default to Google Maps API for backwards compatibility, but would allow people to select an alternative provider with its own API key.

Start by reviewing the Mapbox Geocoding API documentation linked in the issue and comparing its authentication and response model with the existing Google Maps provider. Done means adding Mapbox as an optional provider with its own API key while preserving Google Maps as the default; the repository entry points and tests are not identified in the issue.

Written by the indexing model from the issue text.