feat: Conversation update API for sla_policy_id (#8973)

- Add an endpoint for updating conversation attributes (priority / sla_policy_id )
- Swagger spec
- minor chores around the conversation API/spec

Fixes: https://linear.app/chatwoot/issue/CW-2100/feat-backend-api-to-update-the-sla-of-a-conversation

config/routes.rb

@@ -78,7 +78,7 @@ Rails.application.routes.draw do
           namespace :channels do
             resource :twilio_channel, only: [:create]
           end
-          resources :conversations, only: [:index, :create, :show] do
+          resources :conversations, only: [:index, :create, :show, :update] do
             collection do
               get :meta
               get :search