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