Legend API enhancements

[davenquinn]

We have a new API that allows a legend to be generated for a specific map area/zoom level:

• https://dev.macrostrat.org/api/v3/map/carto/legend?bounds=-100,43,-98,44&zoom=8
• OpenAPI documentation

We have similar APIs in v2 but these only work for individual map sources, or single points.

It would be great to add more complete lithologies, intervals, etc. to the response, to convey more information for each unit without additional API queries.

[davenquinn]

Here's an example of the legend API in use:
https://dev.macrostrat.org/docs/web-components/iframe.html?id=map-interface-map-legend--utah&viewMode=story