Make serving config optionally dynamic

This adds the ability to switch to a different serving config from the
default one using a `serving_config` query parameter.

This will allow us to build a feature in Finder Frontend for internal
users to preview changes to serving configs.

app/services/discovery_engine/query/search.rb

@@ -56,7 +56,9 @@ def query
     end
 
     def serving_config
-      ServingConfig.default.name
+      return ServingConfig.default.name unless query_params[:serving_config].present?
+
+      ServingConfig.new(query_params[:serving_config]).name
     end
 
     def page_size

spec/services/discovery_engine/query/search_spec.rb

@@ -204,6 +204,16 @@
           )
         end
       end
+
+      context "when a serving config is manually specified" do
+        let(:query_params) { { q: "garden centres", serving_config: "preview" } }
+
+        it "calls the client with the expected parameters" do
+          expect(client).to have_received(:search).with(
+            hash_including(serving_config: ServingConfig.new("preview").name)
+          )
+        end
+      end
     end
   end
 end