feat: Add route to list accounts that belongs to a platform_app (#12140)

This PR creates a new route to list all accounts that a platform_app has access to.

Fixes: #12109

config/routes.rb

@@ -425,7 +425,7 @@ Rails.application.routes.draw do
         resources :agent_bots, only: [:index, :create, :show, :update, :destroy] do
           delete :avatar, on: :member
         end
-        resources :accounts, only: [:create, :show, :update, :destroy] do
+        resources :accounts, only: [:index, :create, :show, :update, :destroy] do
           resources :account_users, only: [:index, :create] do
             collection do
               delete :destroy