Add `Duo Chat: Unique Users` panel to AI impact dashboard

What does this MR do and why?

Adds the Duo Chat: Unique users with the existing duo_pro_usage_rate_over_time visualization, and updates the existing Duo seats: Assigned and used panel to a new duo_chat_addon_usage visualization.

When Duo seats: Assigned and used was added, it was incorrectly given the data meant to be rendered in Duo Chat: Unique users. Now that we're adding the 2nd panel, we are updating it to render the expected data. See thread for more info.

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Before	After

with insufficient permissions

How to set up and validate locally

1. Ensure that you are using GitLab Ultimate
2. Configure your GDK to use ClickHouse

# enable clickhouse
Feature.enable(:clickhouse_data_collection)
Feature.enable(:event_sync_worker_for_click_house)

# enable the worker to sync code suggestions events to clickhouse
Feature.enable(:code_suggestion_events_in_click_house)

3. Seed AI analytics data: FILTER=ai_usage_stats bundle exec rake db:seed_fu
4. Create a new group
5. Navigate to the Analytics dashboard page for the new group (ex. http://gdk.test:3000/groups/flightjs/-/analytics/dashboards)
6. Select the AI impact analytics dashboard

To mock the purchased seats for the Duo seats: Assigned and used panel, you can mock the GraphQL resolver with this diff:

diff --git a/ee/app/graphql/types/gitlab_subscriptions/add_on_purchase_type.rb b/ee/app/graphql/types/gitlab_subscriptions/add_on_purchase_type.rb
index 0ec750f25164..79715da904c2 100644
--- a/ee/app/graphql/types/gitlab_subscriptions/add_on_purchase_type.rb
+++ b/ee/app/graphql/types/gitlab_subscriptions/add_on_purchase_type.rb
@@ -21,9 +21,12 @@ class AddOnPurchaseType < Types::BaseObject
 
       alias_method :add_on_purchase, :object
 
+      def purchased_quantity
+        200
+      end
+
       def assigned_quantity
-        context[:assigned_add_on_counts] ||= {}
-        context[:assigned_add_on_counts][add_on_purchase.id] ||= add_on_purchase.assigned_users.size
+        150
       end
 
       def name

Related to #468304 (closed)