[native_assets_cli] Remove OS and architecture from `CodeAsset`

[dcharkes]

CodeAssets occur in an input or an output belonging to an input. We could simply remove the OS and architecture from the assets, because they must have the OS/architecture that is in the code config.

[dcharkes]

Notes from discussion with @mkustermann:

• Do we need to consider future support for multi-architecture code assets before removing the OS/arch as output?
• 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.

[dcharkes]

The fields are no longer being red by newer hooks/SDKs.

They are still written for backwards compatibility, we can clean this up in a couple of weeks.