feat: add upload-release-to-gcs.sh script

[ValentaTomas]

Adds scripts/upload-release-to-gcs.sh — downloads firecracker-{amd64,arm64} assets from a GitHub release and uploads them to GCS at the correct arch-subdir layout. Existing objects are never overwritten. Supports --dry-run.

./scripts/upload-release-to-gcs.sh --tag v1.14.1_f0a35a1 --bucket my-bucket --dry-run
./scripts/upload-release-to-gcs.sh --tag v1.14.1_f0a35a1 --bucket my-bucket

[cursor]

PR Summary

Low Risk
Low risk because this only adds a standalone shell script and doesn’t change runtime code paths; main risk is operational misuse of gcloud storage cp/permissions when run.

Overview
Adds scripts/upload-release-to-gcs.sh to download firecracker-amd64/firecracker-arm64 assets from a GitHub release via gh and upload them to gs://<bucket>/<tag>/<arch>/firecracker, skipping uploads when the destination object already exists and supporting a --dry-run mode.

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