[native_assets_cli] Provide guidance on name spacing extensions

[dcharkes]

We should provide guidance on name-spacing the extensions to the base protocol so that we don't end up with name clashes.

This is for:

1. type in EncodedAsset, and
2. the key Config.

I believe the most natural way to name-space them is by using package names.

For the type in encoded assets and build_asset_types:

• code_assets/code_asset
  • or code_assets/code
  • or code_asset in package:code_asset
• code_assets/multi_architecture
• data_assets/data
  • or data_asset in package:data_asset
• flutter/font
  • or flutter_assets/font
• unity/shader
• ...

And in the config extensions simply always the package name.

This should prevent any name clashes between extensions and between extensions.

Together with #2091, this should prevent name clashes between the base protocol and extensions.

Thanks for the suggestion @mkustermann 🙏

[dcharkes]

With #2189, the asset-types are written as namespaced as well.

The asset-types and config keys are still read as the old ones s well, we can clean this up later.