Implement the basic business logic endpoint for creating invoices
- Obtain a FOLIO invoice number from GET /invoice-storage/invoice-numbers - these are always system generated.
- Calls POST /invoice-storage/invoices to persist the invoice
- request validation should be performed - via annotation is adequate for now
- Unit test coverage on new code is at or above 80%
- API tests have been updated
- the POST /invoicing/invoices has been implemented according the description above