[fuchsia] Place FFI callback stub into the package.

[rmacnak-google]

This is the Flutter side of https://dart-review.googlesource.com/c/sdk/+/495840. The Flutter side should be submitted first, i.e., make the blob available before trying to use it.

Most of the diff is making explicit that previously existing resources go under /pkg/data so that the new resource can go under /pkg/lib like ELF shared libraries do.

[gemini-code-assist]

Code Review

This pull request introduces FFI callback stub generation for Fuchsia Dart and Flutter runners and refactors resource destination paths to explicitly include the 'data/' prefix. Review feedback suggests adding a parameter assertion to the new GN template for consistency, simplifying redundant string interpolation, and including the '--enable_mirrors=false' flag to align the Flutter runner's stub generation with the Dart runner's implementation.