Add facet to query and aggregate based on the work_item_type

Problem to solve

We want capability to filter and aggregates issues based on work_item_type_id for the new UI.

Proposal

We can add a param work_item_type_id and frontend can make a call for each work_item_type and can get the aggregations as well as filter them by the scope. I have added a small patch for the approach

diff --git a/app/services/concerns/search/filter.rb b/app/services/concerns/search/filter.rb
index 718318cb9124..596a221ce4ba 100644
--- a/app/services/concerns/search/filter.rb
+++ b/app/services/concerns/search/filter.rb
@@ -10,7 +10,8 @@ def filters
         confidential: params[:confidential],
         include_archived: params[:include_archived],
         num_context_lines: params[:num_context_lines]&.to_i,
-        hybrid_similarity: params[:hybrid_similarity]&.to_f
+        hybrid_similarity: params[:hybrid_similarity]&.to_f,
+        work_item_type_id:  params[:work_item_type_id]&.to_i
       }
     end
   end
diff --git a/ee/lib/gitlab/elastic/search_results.rb b/ee/lib/gitlab/elastic/search_results.rb
index a0cefb7968c8..4d70f7e19e55 100644
--- a/ee/lib/gitlab/elastic/search_results.rb
+++ b/ee/lib/gitlab/elastic/search_results.rb
@@ -380,7 +380,7 @@ def work_item_scope_options
           {
             klass: Issue, # For rendering the UI
             index_name: ::Search::Elastic::References::WorkItem.index,
-            not_work_item_type_ids: [::WorkItems::Type.find_by_name(::WorkItems::Type::TYPE_NAMES[:epic]).id]
+            work_item_type_ids: [filters[:work_item_type_id]]
           },
           filters.slice(:order_by, :sort, :confidential, :state, :label_name, :include_archived, :fields)
         )

Adding few screenshots for the scopes

scope	screenshot
issue
requirement
task
incident