[native_assets_cli][native_assets_builder] CLI input and output composability

[dcharkes]

The Dart API can be revved independently of the protocol, bumping this out of v1.0.

@mkustermann suggested that instead of having JSON/YAML -> Dart objects for BuildConfig and Dart objects -> JSON/YAML for BuildOutput to use extension methods (or extension types).

This would enable new launchers (non Dart-standalone, non Flutter) to define new asset types in a package.

JSON/YAML -> Dart objects for BuildConfig

This is already kind of composable with multiple packages, as multiple objects could be wrapped around JSON/YAML.

Dart objects -> JSON/YAML for BuildOutput

This is currently not composable (users would have to manually 'merge' the Maps/Lists).

Extension methods working only on the underlying data would modify the json on every edit, rather than serializing at the end.

We would use a different 'view' for Data assets, Native code assets, Wasm code assets, proguard rules assets.

@mosuem WDYT?

[dcharkes]

This is done. Thanks @mkustermann!