[native_assets_cli] JSON nest encoding of assets

[dcharkes]

Currently, the asset encoding is that type is a special key in the map and all the other keys are the encoding.

This is a bit smelly, because it means that if someone were to make a type in the encoding it would break.

Also it leads to some weird filtering in maps:

native/pkgs/native_assets_cli/lib/src/encoded_asset.dart

Line 23 in 2dba4f5

if (key != _typeKey) key: json[key],

@mkustermann suggested we should nest it.

{
  "type": "code_asset",
  "encoding" : {
    "architecture" : "x64"
  }
}