Product updates: L40Ss, proxy auth tokens, and sandbox disk snapshotting

What this post added

Introduced a Filesystem API for Sandboxes to seamlessly read and write files, and added disk snapshotting capabilities to create, branch, and restore Sandbox states, eliminating cold-start times. Sandboxes also now support `fsnotify-like` file watching and accept larger write payloads up to 1 GiB.