Spatial index widgets

[juandjara]

related story: https://app.shortcut.com/cartoteam/story/454268/adapt-carto-api-client-to-be-able-to-work-with-dynamic-spatial-index

[donmccurdy]

Thanks, this is nicely structured! A few comments/questions, still working my way though the code review. :)

[donmccurdy]

@juandjara great work here! I think all that's left after #41 is the one other comment above, removing the spatialFiltersResolution from the function options and always computing it internally when a spatial filter is used.

As the next step, should we publish this as an alpha release so that external examples can be built on it? If you're ready, feel free to do that any time. The steps would:

1. Update package.json#version to 0.4.1-alpha.0
2. Run yarn postversion
3. Manually run the release action, selecting this branch as the target, as described in CONTRIBUTING.md