[FEATURE] atelet fetches public sandbox assets over HTTPS #3672

atelet keeps an anonymous GCS client to download public sandbox assets from gs:// URLs, and the bundled gVisor SandboxConfig points at gs://gvisor. Replace the client with a generic HTTPS fetcher and HTTPS default URLs.

Tracks "Fetch public sandbox assets over HTTPS" in the Provider-Neutral Core workstream of the Extensible Install Roadmap.

Current state (0b635b80)

• Anonymous GCS client: cmd/atelet/main.go. gs:// fallback: cmd/atelet/sandbox_assets.go.
• Default URLs gs://gvisor/...: manifests/ate-install/sandboxconfig-gvisor.yaml. Example URLs: docs/reference/api-guide.md.
• Private assets already go through the object-store plugin (Fetch private sandbox assets through the object-store plugin #2439, fix(atelet): fall back to anonymous GCS read with objectstore-plugin Internal #2614). Design: how should object-store plugins signal that atelet may read a public sandbox asset anonymously? #2630 is the open design question on how a plugin signals that an asset may be read anonymously.

Required changes (two steps)

1. Add an HTTPS fetcher for public assets, with SHA-256 still verified. Switch the default gVisor URLs to HTTPS (https://storage.googleapis.com/gvisor/...). Deprecate the anonymous gs:// fallback with a warning.
2. Remove the anonymous GCS client and the gs:// fallback. gs:// assets are served only by a GCS-capable plugin.

Dependencies: the outcome of #2630.

Breaking change: step 2, for SandboxConfigs that still use public gs:// URLs without a GCS plugin.

Enforcement

• Unit tests for HTTPS fetch and checksum mismatch.
• atelet has no GCS import after step 2.
• The default kind install runs gVisor actors with no GCS-capable plugin.

Parent Issue: #3161