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

enterprise/app/models/sla_policy.rb

@@ -22,6 +22,7 @@ class SlaPolicy < ApplicationRecord
   validates :name, presence: true
 
   has_many :conversations, dependent: :nullify
+  has_many :applied_slas, dependent: :destroy
 
   def push_event_data
     {