[vizbuilder] support the "is_required" annotation

[davelandry]

If a dimension has the is_required: true annotation, and the user is not already drilling down by one of the dimension's levels, then add the top-most level to the mondrian-rest-client drilldowns.

[frabarz]

I wrote this in a way it adds the level internally, but there's no indication of this drilldown in the user interface. Should I make a group appear and block the button to delete it instead?

[davelandry]

@frabarz nope, let's do internally and hidden!