☂️ [native_assets_cli] Protocol overhaul

[dcharkes]

Issue to track issues that have to do with the JSON protocol of hook/build.dart.

• [native_assets_builder] build input and output should be JSON instead of YAML #991
• [native_assets_cli] JSON protocol nest per asset type #993

Dart API umbrella issue:

• ☂️ [native_assets_cli] Overhaul dart API for build.dart #882

[dcharkes]

The remaining issue is tracked in an issue already.