Create Invoice api call (POST /invoices) - Allow assigning products rather than explicitly defining products for each call.

<!-- Before posting please check our "Troubleshooting" category in the docs: --> Setup • Version: v5.8.19 • Environment: Self-hosted Interface • Flutter: [] • React: [] • Both: [] • API [X] Checklist • Can you replicate the issue on our v5 demo site or No • Have you searched existing issues? Yes • Have you reported this to Slack/forum before posting? Yes • Have you inspected the logs in storage/logs/laravel.log for any errors? Yes, none found Describe the bug When creating invoices using the API, it should be enough to just specify the product_key and quantity in the line_items array. Currently we are required to specify cost because if cost is not specified the line items are shown with $0 creating invoices with $0 due! Steps To Reproduce Here is the body for the API call which should work: Here is what is required currently (for invoices with non-zero amounts!) Expe…