[core] Flutterfire should use "Pigeon"

[iapicca]

Hi all,
Whether or not this proposal on the flutter repository will be received,
I believe that flutterfire's plugins will benefit from
using protobuf as default for platform channel communication,
preventing the following:

• FlutterStandardTypedData issues
• often require to cast data
• frequent lost of type-safety
• lost of performance with List, Map<String,dynamic>
• client error received as platform error

Thank you

[iapicca]

In light of this answer and to keep consistency with flutter's first party plugins
Flutterfire plugins should use pigeon to handle communications between native and dart
Thank you