draft: offline legacy observers

[Gnative]

Description

Adds download observers for OfflineLegacyModule maps which includes timeouts and throwing errors for max tiles exceeded.

I have this in production for a few months via my own branch, and it all works without any issues..
It still needs a little bit of work to bring this into the project, tests etc. I'm a little busy to complete it at the moment but wanted to add this as a PR incase someone else is looking for this behaviour.

There is a new OfflineLegacyExample to work against example/src/examples/CacheOffline/OfflineLegacyExample.ts

Checklist

• I've read CONTRIBUTING.md
• I updated the doc/other generated code with running yarn generate in the root folder
• I have tested the new feature on /example app.
  • In V11 mode/ios
  • In New Architecture mode/ios
  • In V11 mode/android
  • In New Architecture mode/android
• I added/updated a sample - if a new feature was implemented (/example)