Add aggregation counts to chip keys and values

The best possible description of why this is important is in here: https://github.com/guardian/grid/pull/4252.

Aggregation counts should be present next to chip keys and values in dropdowns.

They should be narrowed to the current search.

Start by reading the linked Guardian Grid pull request for the rationale, then trace the chip key and value dropdowns and how the current search is represented. Done means showing aggregation counts beside both keys and values, with counts narrowed to the current search.

Written by the indexing model from the issue text.