[Feature Request]: Make us of non-administrative boundary files

[wadhwamatic]

Provide a clear and concise description of what you want to happen.

In some cases, there can be boundary files that are not administrative boundaries. This includes groupings such as agro-ecological zones, livelihood zones, or watersheds / drainage basins.

As a PRISM user, I want to be able to:

1. Optionally visualize boundaries / polygon layers that are not displayed by default so that I can see climate layers in relationship to locations beyond administrative areas
2. Create zonal stats using the run analysis feature for non-administrative area polygons

During the PRISM configuration process, I want to be able:

1. Add a new layer with type boundary in layers.json using the same process as admin boundary layers
2. In prism.json, add a new set of layers called additionalLayers which is an array of layers that are hidden from the UI until a user activates them

One suggestion to show these layers in the UI is to place them at the bottom of the legend section. I think this is clearer than adding them to the left side panel since they are non-standard layers. Here's a quick mockup:

Is there anything else you can add about the proposal? You might want to link to related issues here, if you haven't already.

I have added a basin_boundaries layer as type boundary here: #1247