feat: Add standalone outgoing messages count API endpoint (#13419)

This PR adds a new standalone `GET
/api/v2/accounts/:id/reports/outgoing_messages_count` endpoint that
returns outgoing message counts grouped by agent, team, inbox, or label.

config/routes.rb

@@ -446,6 +446,7 @@ Rails.application.routes.draw do
               get :bot_metrics
               get :inbox_label_matrix
               get :first_response_time_distribution
+              get :outgoing_messages_count
             end
           end
           resource :year_in_review, only: [:show]