[code_assets] Debug symbols

[dcharkes]

We should investigate how/whether debug symbols can/should be outputted separately from the dylib for the various places an app is uploaded.

If yes, we need to consider how to add such information to CodeAssets in the build hook protocol.

This could be useful for keeping app size small, but still being able to get a symbolized stack trace from a crash on a user device. (N.b. afaik Dart itself does not upload the debug symbols in the app, but has them in the SDK somewhere, so if you have an crash with a non-symbolized stack trace and you know the Dart version, you can get it symbolized locally. @sstrickl might be able to add some more context.)

Context:

• 3.x release prevents uploads to macOS App Store simolus3/sqlite3.dart#333 (comment)