Use custom order IDs in create intent request

[Mayisha]

Fixes #3397

Custom order IDs are ignored in the create intent request when the new checkout experience is enabled.
This error does not happen in the legacy checkout experience because in the legacy experience get_order_number method is used in the metadata ( which is hooked to the woocommerce_order_number filter).

Changes proposed in this Pull Request:

In this PR, we are using the get_order_number instead of get_id for in the metadata of create intent request.

Testing instructions

1. Install any plugin for custom order IDs (e.g Custom Order Numbers for WooCommerce which is used by the reporter)
2. Update the plugin's settings installed in the previous step to use custom order IDs. ( I have used #Test - prefix in my test below)
3. In your Stripe settings page enable the new checkout experience.
4. Enabled logs for the Stripe plugin.
5. Now as a shopper, add a product to your card and complete the checkout with a credit card.
6. Go to WooCommerce > Status > Logs and open the log file which includes the recent order logs.
7. Find the log for payment_intents request.
8. In develop notice the metadata field. Notice that it is sending the actual database order ID instead of the custom pre-fixed order ID.

9. Now checkout to this branch and as a shopper purchase a product again.
10. Find the log for payment_intents request for this order.
11. Notice that now it is sending the custom pre-fixed order ID in the meta data.

[wjrosa]

Nice fix! Code is good and works as expected: