[native_assets_cli] `BuildOutput` API

[dcharkes]

It seems that this BuildOutput doesn't make a real API (such as output.addAsset(...)) instead it exposes it's constituent fields. Some of those fields like assets are mutable, others are not mutable.

It may be better to provide an API that one can use to add things to the output. The API can be hierarchical just like the config.

What should this API look like?
And should we separate API from implementation?

Related discussions:

• Initial version #3 (comment)

[dcharkes]

One thing to do here is to make a clean API, rather than exposing the fields.

[dcharkes]

@mosuem as promised: This is the API that requires some iteration. 😃