feat: Assignable agents API (#2131)

`inboxes/id/assignable_agents` returns all users that have permission to a conversation happening in that inbox.

config/routes.rb

@@ -88,6 +88,7 @@ Rails.application.routes.draw do
           end
 
           resources :inboxes, only: [:index, :create, :update, :destroy] do
+            get :assignable_agents, on: :member
             post :set_agent_bot, on: :member
           end
           resources :inbox_members, only: [:create, :show], param: :inbox_id