feat(api): add GCP external storage support (#3943)

## Summary

Add typed GCP provider support for organization external storage. Expose
retry headers on video creation errors when available.

## Changes

- Export the new GcpExternalStorageProvider response model with bucket,
region, audience, workload identity, and provider type fields.
- Allow GCP providers in external storage creation parameters,
configurations, and audit log registration events.
- Add optional headers to VideoCreateError as a string mapping for
Retry-After and Retry-After-Ms values.

Co-authored-by: jbeckwith-oai <269036200+jbeckwith-oai@users.noreply.github.com>