[native_assets_cli] Open ended asset types list

[dcharkes]

@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)?