Scanning multiple documents at once - edge function

[michalstruck]

We should enable users to scan multiple images at once (long raports, multipage invoices).
Stuff to consider

• incoming requests will contain an array of base64 encoded images,
• quantity/price merge logic should remain unchanged, if double items occur on the same image. If we have multiple equally named items on different images, the first occurrence should take precedence, and not be overwritten by subsequent scan results,
• we should request all data from azure at once, but in separate requests - at least for now.