Adding OGC Base Maps from other endpoints (ArcGIS, & Open Standard)

**Problem Statement**
Currently kepler.gl is a powerful and light geospatial application, however cannot be utilised without a MapBox service. This means that the application is only usable in situation where an internet connection can be established. Not being able to access base maps from other service endpoints limits potential use cases where a user might want customised base mapping sourced from a variety of locations (locally from the machine or potentially from a different cloud-based server or instance).

**Proposed Solution**
The functionality to set base mapping layers from different endpoints (not just MapBox) and also through the python module for kepler.gl would extend kepler.gl functionality to operate across disconnected or isolated network instances and also allow for different user experiences with custom foundation layers.

**Current Alternatives**
Currently I use folium/leaflet which lacks the geo analytics and user experience of kepler.gl but can support OGC and ArcGIS web mapping layer inputs. I work for a company that hosts custom isolated layer services due to being regularly disconnected. Currently our only other option is to use expensive enterprise GIS solutions which are overkill but can consume the services that are currently hosted.