Support platform specific default-flavor

[prateekmedia]

Use case

Currently the pubspec.yaml's default-flavor assumes that the flavor name exists in all platforms that support flavor, which might be a wrong assumption.

For example in the app I am working on, it only has flavors for android and not for iOS, which works for us. But we also want default-flavor, but we can't have it since it assume that flavor exists on iOS and flutter run fails.

Also in iOS we have schemes and not flavors. Schemes are generally in PascalCase and not in snake_case or camelCase. So someone might want a pascal case scheme name for iOS and camelCase for android.

Proposal

Allow default-flavor-$platformName as a parameter if it is present else use default-flavor, this way if someone only has flavor's on android, they can just do:
default-flavor-android: $flavorName

Or do:
default-flavor-android: $flavorName1
default-flavor-iOS: $flavorName2

[tirth-patel-nc]

@prateekmedia thanks for the report. labelling it as a new proposal.

#22856 is a related issue and already supported, but it is not tied to any specific platform https://docs.flutter.dev/tools/pubspec#default-flavor-field

[bkonyi]

This seems reasonable to me at a glance. @reidbaker @vashworth, WDYT?