Implementation of invoice issuance when goods have been shipped from merchant

[DrStep]

Feature description:

• to issue an invoice when goods have been shipped, firstly purchase order must be created while buyer makes checkout on the merchant's portal,
• once merchant notifies Billie's platform about shipment, based on original order (connecting is implemented via order_id) invoice is prepared and returned as a response, so that buyer can get it through merchant platform
• then we are waiting for the payment [this logic will be implemented further]

Added during development:

• tests related to invoice issuing feature
• logic implementation that resolves all tests
• SQL scripts working with Flyway for simple migrations
• opportunity to run all services with single docker compose up command