Add an endpoint for processing pay-for-order orders

[hsingyuc]

Is your feature request related to a problem? Please describe.

Currently, WooCommerce Blocks does not support pay-for-order checkouts. The second step in this process is to add an endpoint to the store API to process pay-for-order orders.

Describe the solution you'd like

Pass the order ID and key to the checkout endpoint so we can checkout with the order and easily validated by the endpoint.

Describe alternatives you've considered

To process the pay-for-order orders inside of the existing checkout endpoint, but this might require a lot of conditional checks and combining the logic for cart and orders can get messy.

Additional context

Slack - p1681808113297559/1680686053.947699-slack-C8X6Q7XQU