[native_assets_cli] Open ended asset types list

@mkustermann suggested that `BuildInput` should provide a list of support asset types.
Providing any assets in the `BuildOutput` that are not supported is an error.

This would enable other embedders/launchers to support their own asset types, without making PRs to this package.

Some open questions:

* How are such assets versioned? If such asset type wants to do a breaking change, it wouldn't be covered by the protocol version. Leading to runtime failures.
* How should asset types be name spaced? How do we avoid name collisions on a `String type`? Should embedders/launchers register a package according to their name on dartdev and use that (or prefix with that)?