Consider: Batch initial worker synchronization into a single tar archive #3931

Description

The initial workspace synchronization currently sends each directory and file to the worker through separate, sequential RPCs. Large projects therefore require many round trips before pub get can start.

Use the existing Workspace.importTarArchive API to send the initial workspace as one tar archive. Keep individual file updates for subsequent edits.

But lets if its worth it because the frontend sometimes generates a pubspec_overrides.yaml or pubspec.yaml and the user could make edits while the worker is starting. Sending the original download archive alone would miss these changes.