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

app/models/conversation.rb

@@ -24,6 +24,7 @@
 #  contact_inbox_id       :bigint
 #  display_id             :integer          not null
 #  inbox_id               :integer          not null
+#  sla_policy_id          :bigint
 #  team_id                :bigint
 #
 # Indexes
@@ -294,3 +295,5 @@ class Conversation < ApplicationRecord
     "NEW.display_id := nextval('conv_dpid_seq_' || NEW.account_id);"
   end
 end
+
+Conversation.include_mod_with('EnterpriseConversationConcern')