[native_assets_cli] Introduce `XCFrameworkAsset`

[dcharkes]

CodeAssets are defined to be (1) single architecture, and (2) not signed, and (3) not packaged.

For some use cases, it might be useful to report assets that are pre-signed and pre-packaged. So that Flutter, Dart, and other SDKs don't have to tinker with it.

This would require adding dedicated support for such frameworks in Flutter and Dart. Marking low prio.

(I believe I had surfaced this idea before, but can't find a tracking issue.)