feat: Contactable Inboxes API (#2101)

- Add endpoint which lists inboxes through which a contact can be contacted
- Conversation creation API auto-creates contact_inbox for specific channels [ Twilio, email, api]
- Ability to send the initial message payload along with the conversation creation
- Fixes #1678 ( issue saving additional attributes for conversation )

config/routes.rb

@@ -69,6 +69,9 @@ Rails.application.routes.draw do
               get :search
               post :import
             end
+            member do
+              get :contactable_inboxes
+            end
             scope module: :contacts do
               resources :conversations, only: [:index]
               resources :contact_inboxes, only: [:create]