[firebase_analytics] Add method channel to get App Instance ID.

[MauScheff]

I'm trying to access the App Instance ID as described here (JSON Body):

https://developers.google.com/analytics/devguides/collection/protocol/ga4/sending-events?client_type=firebase#required_parameters

I think it would be best to access this from this library since it's bound with the native Firebase Analytics libraries. Is there any chance that this can be added? Or a a recommended way to approach this?

Thank you,
Mauricio