chore: Add sla policy association to conversation (#7360)

Adds the sla policy association to the conversation

Fixes: https://linear.app/chatwoot/issue/CW-1615/applying-an-sla-to-the-conversation

enterprise/app/models/enterprise/enterprise_conversation_concern.rb

@@ -0,0 +1,7 @@
+module Enterprise::EnterpriseConversationConcern
+  extend ActiveSupport::Concern
+
+  included do
+    belongs_to :sla_policy, optional: true
+  end
+end

enterprise/app/models/sla_policy.rb

@@ -18,4 +18,6 @@
 class SlaPolicy < ApplicationRecord
   belongs_to :account
   validates :name, presence: true
+
+  has_many :conversations, dependent: :nullify
 end