New attack technique: Inject a Malicious Startup Script into a Vertex AI Workbench Instance (gcp.execution.modify-vertex-notebook-startup)

[Minosity-VR]

What does this PR do?

New attack technique: gcp.execution.modify-vertex-notebook-startup

Motivation

GCP parity with existing AWS attack techniques.

Test results

• stratus detonate gcp.execution.modify-vertex-notebook-startup
• google.cloud.notebooks.v2.NotebookService.UpdateInstance appears in GCP Admin Activity audit logs (no audit log observed — may require non-default audit config for Notebooks DATA_WRITE events)

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

[christophetd]

Error: Error creating Instance: googleapi: Error 403: Notebooks API has not been used in project security-detection-reseach before or it is disabled. Enable it by visiting https://console.developers.google.com/apis/api/notebooks.googleapis.com/overview?project=security-detection-reseach then retry. If you enabled this API recently, wait a few minutes for the action to propagate to our systems and retry.

I think we need to mention this in the docs, or ideally print a nicer error message

[christophetd]

Note: this PR was inadvertently merged into the stacked base branch (#797's branch) rather than main, so its changes are not actually in main. Recreated as #845 against main.