Chore: API Improvements (#2956)

- API to fetch info of a single inbox
- Document passing custom_attributes in the API
- Ability to filter contacts with contact identifier in search API

config/routes.rb

@@ -97,7 +97,7 @@ Rails.application.routes.draw do
           end
           resources :custom_attribute_definitions, only: [:index, :show, :create, :update, :destroy]
           resources :custom_filters, only: [:index, :show, :create, :update, :destroy]
-          resources :inboxes, only: [:index, :create, :update, :destroy] do
+          resources :inboxes, only: [:index, :show, :create, :update, :destroy] do
             get :assignable_agents, on: :member
             get :campaigns, on: :member
             get :agent_bot, on: :member