Support colormap configuration for imagery coloring

[philvarner]

Currently, a colormap_name can be specified for a collection, which uses one of the pre-defined colormaps. However, this doesn't work for data that has a few discrete values, like land classification. TiTiler supports a colormap that takes a JSON string representing the value->color mapping. Support configuring and passing this in filmdrop-ui.

See https://developmentseed.org/titiler/advanced/rendering/ for info on this parameter