In content editor, support multiple observerIds for a dropdown

In the document editor (content perspective only), it's possible to create chained dropdowns using observableId/observerId configuration, see https://documentation.bloomreach.com/content/v14/docs/selections-plugin-configuration

Currently supported use case:
Populate one value list based on the selected value of another, e.g. a City dropdown repopulating based on a Country dropdown value selection.

The improvement request is to support dropdowns listening to multiple other dropdowns.

Requested use case: 
Populate one value list based on the selected value of multiple other dropdowns, e.g. a City dropdown repopulating based on both a Country dropdown value selection and a State dropdown value selection.