[native_toolchain_rust] Introduce a package:native_toolchain_rust

[dcharkes]

We should consider introducing a package to help calling the Rust compiler from Dart code.

Things this package could contain:

• [ ] A mapping from native_assets_clis Target and IOSSdk to Rust target triples (These seem to deviate from other toolchains. Apple clang: arm64-apple-ios-simulator vs Rust aarch64-apple-ios-sim.
• [ ] (Feel free to edit and add wishes.)
• [ ] ...

This package could be used in multiple ways:

1. Running the Rust compiler from the user machine inside build.dart scripts.
2. Running the Rust compiler from some tools/compile_all.dart script on the GitHub CI (after which the artifacts can be downloaded in a build.dart script).

cc @fzyzcjy @robertbastian Please post any wishes you have here.