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

app/controllers/platform/api/v1/accounts_controller.rb

@@ -1,4 +1,11 @@
 class Platform::Api::V1::AccountsController < PlatformController
+  def index
+    @resources = @platform_app.platform_app_permissibles
+                              .where(permissible_type: 'Account')
+                              .includes(:permissible)
+                              .map(&:permissible)
+  end
+
   def show; end
 
   def create