Flutter in app purchase - firebase functions backend

I've submitted PR for original codelab but meanwhile maybe it'll help someone. This is just updated and working version of codelab example codelab iap

Changes made

Upgraded to node 18
Updated packages
Switched to ESM modules
Fixed imports
Updated deploy functions that didn't work

How to use it (original codelab instructions)

https://codelabs.developers.google.com/codelabs/flutter-in-app-purchases#8