Adjust legend label to indicate whether layer is visible or not

[shirlysteph]

Description: Adjust the look of labels in the legend, based on whether the feature layer is visible on the map or not. This adjustment can be a change in label transparency, grey fill etc.

Context: This feature is an improvement to the "Filter by Property" feature (see issue #1770). The filter feature currently allows the user to toggle the visibility of vector features on the map based on filterable properties. For example, the Permafrost Zones layer can be filtered and their visibility adjusted based on various zone types. The values for some of these properties (e.g., zone types) are configured in the legend using the color palette set in the portal configuration document. The current request is to indicate/highlight within this legend panel whether a layer is visible in the map or not.