feat: SLA CRUD APIs (EE) (#7027)

Fixes: https://linear.app/chatwoot/issue/CW-1613/sla-api

Co-authored-by: Sojan Jose <sojan@pepalo.com>

db/migrate/20230503101201_create_sla_policies.rb

@@ -0,0 +1,13 @@
+class CreateSlaPolicies < ActiveRecord::Migration[6.1]
+  def change
+    create_table :sla_policies do |t|
+      t.string :name, null: false
+      t.float :frt_threshold, default: nil
+      t.float :rt_threshold, default: nil
+      t.boolean 'only_during_business_hours', default: false
+      t.references :account, index: true, null: false
+
+      t.timestamps
+    end
+  end
+end