Show a legend for data layers that require one in the Cesium View

[robyngit]

The legend could be created automatically based on which ever feature is colour-coded, or it could be a static image that is provided along with the configuration for the data layer

[robyngit]

Related: #1888, #1768, #1886

[robyngit]

An example legend based on Ingmar's lake change dataset (PermafrostDiscoveryGateway/pdg-portal#28)

[robyngit]

Some feedback we received recently in reference to the mini-legend we have in place now:

Legend not intuitive, little hard to read and interpret; what is the intended user group?; even if scientists (only) I think the legend should be clearer

[robyngit]

This feature has been requested by DRP, from duplicate issue #2239:

Describe the feature you'd like
The legend should be larger and should support more than 6 colors. I think the way to do that is to expand the entire legend in a tooltip-like object rather than one color at a time, which would allow more than 6 to be defined in categorical color palettes, and would increase the visibility of the colors.

Is your feature request related to a problem? Please describe.
We got some strong feedback at the DRP meeting that the legends are hard to read. I think this may be partially attributable to the fact that the DRP project has heavy Cesium utilization and that we have several layers whose color palettes are not well defined. But given the discussion mbjones robyngit and I had a few weeks back, I think we generally agree that they need to be larger or more readable somehow.

Additional context
I realize that this is not a trivial problem to solve, but I do think it is important for the platform's usability going forward.