New attack technique: Delete All GCS Objects in Batch (gcp.impact.ransomware-gcs-batch-deletion)

[Minosity-VR]

What does this PR do?

Adds a new GCP attack technique gcp.impact.ransomware-gcs-batch-deletion that simulates a ransomware attack by concurrently deleting all objects in a GCS bucket and uploading a ransom note.

Motivation

GCP parity effort with existing AWS techniques (analogous to aws.impact.s3-ransomware-batch-deletion).

Checklist

• The attack technique emulates a single attack step, not a full attack chain
• We have factual evidence & references that the attack technique was used by real malware, pentesters, or attackers
• The attack technique makes no assumption about the state of the environment prior to warming it up

Test results

• Technique detonates successfully
• Detection log identified (storage.objects.delete) — there is no audit log emitted for GCS object deletions without non-default Data Access audit log configuration; a burst-based detection would yield too many false positives given typical GCS usage at scale