Yes, builtin CDN offload is actively under discussion as part of v2 protocol changes, but there is nothing currently available in released versions. The "repo" command that comes with Android dev tools uses the clone.bundle approach (which is basically what we're implementing on git.kernel.org for a handful of core repositories). I would guess "repo" is partly why nobody added anything like this directly to git -- for Google, that particular itch has been scratched.

Anyway, here's hoping that the coming protocol changes will have a native solution to this problem.