[native_assets_cli] `DataConfig` with a target

[dcharkes]

I can see benefits of also exposing the target OS for data assets, but that's a different discussion to be had.

Originally posted by @mosuem in https://github.com/dart-lang/native/issues/2108#issuecomment-2739679481

The DataConfig should have something similar to OS in CodeConfig but it should include web.

Maybe it should be something like:

• Target: web
• Target: native
  • OS: the 5 OSes