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

db/migrate/20230620132319_add_sla_policy_to_conversations.rb

@@ -0,0 +1,5 @@
+class AddSlaPolicyToConversations < ActiveRecord::Migration[7.0]
+  def change
+    add_column :conversations, :sla_policy_id, :bigint
+  end
+end