Internal: b/168395669

From https://dart-review.googlesource.com/c/sdk/+/152427,

At runtime, sometime during isolate startup, an embedder should call Dart_SetDeferredLoadHandler, probably near an existing call to Dart_SetLibraryTagHandler. The callback is given a loading unit id, and should eventually call Dart_DeferredLoadComplete[Error].

We should implement this callback and create endpoints for downloading the necessary files from play store or custom URL.

This is part of the work needed to complete #57617