[code_assets] Support fat binaries

[dcharkes]

Code hooks are invoked for a single architecture and single OS.
And in Flutter for MacOS and iOS we combine the thin binaries into fat binaries.

However, it might be that the user already has a fat binary.

We should consider adding support for reporting fat binaries. We have hypothesized this before:

• If we want to ever add support for fat binaries, we'd probably want to add that as a MultiArchCodeAsset with a MultiArchCodeConfig
  • Which would have multiple architectures in the config.
  • The multi_architecture_code_asset should be listed
  • Must the output contain all requested architectures? Or only a subset?
    • If a subset, then it must be listed in the output.
  • It would be a separate hook invocation.
  • Open questions:
    • How would that work together with the normal code assets? Can these be mixed and matched? Would Flutter for example combine a fat binary with two architectures and and a non-fat-binary with a single architecture if they have the same asset id?
• These MultiArchitectureCodeAssets would likely live in the same package:code_assets such that they can share OS, Architecture and friends.

Originally posted by @dcharkes in #2127

The workaround is to thin the binary from a fat binary in the build hook:

lipo YourFatBinary -extract arm64 -output YourThinBinary_arm64

Related issue:

• [dartdev] [question] Support multi-architecture dylib in native_assets_macos.dart sdk#61130