Adds support for Customer Notes to Custom Order Tables.

As a quick reminder, from a UI perspective, customer notes surface both in the order editor and the checkout page, as in the following screenshot:

Editor screen	Checkout page

Traditionally/under the CPT data-store, these notes have been stored in the post_excerpt field of the wp_posts table.

Closes #34148.

How to test the changes in this Pull Request:

Start by testing migrations.

1. Disable the COT feature (via WooCommerce ‣ Settings ‣ Advanced ‣ Custom Data Stores) and delete your Custom Order Tables (via WooCommerce ‣ Status ‣ Tools).
2. Create some test orders, and set customer notes for some of them.
3. Re-enable the COT feature (but leave the posts table authoritative for the time being), re-generate the tables, and run the migration (wp wc cot migrate).
4. Once the migration completes, inspect the wp_wc_orders table. You should see a customer_note column and the value of this column should match what you see in the post_excerpt column for the corresponding rows from the wp_posts table:

Ensure we can still edit customer notes.

1. At this stage, the posts table should still be authoritative. Also check that Keep the posts table and the orders tables synchronized is enabled (both these things are managed via WooCommerce ‣ Settings ‣ Advanced ‣ Custom Data Stores).
2. Edit one of the orders then edit the customer note. Save/update.
3. The change should have persisted.
4. Now change the authoritative tables to COT and repeat the test (since a change was made to the meta box templates—which are shared by both the CPT and COT editors—I want to check editor-based updates of this field work for COT as well as CPT).

Now test the COT data-store as a customer.

1. With COT enabled (again: COT tables should be authoritative, synchronization with posts should be enabled) visit the storefront and place an order.
2. On the checkout screen, specify a Customer Note. Place the order.
3. As site administrator, edit the order and make sure you can see the Customer Note.
4. Using a DB tool of your choosing, ensure the matching row in wp_posts has the same Customer Note (but, stored in the post_excerpt field).

FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.