[alonsopf89]

Summary

Migrates the `npm` package publishing workflow to **Google OSS Exit Gate** (`go/oss-exit-gate`) using Workload Identity Federation (`google-github-actions/auth@v3`) + Artifact Registry staging (`https://us-npm.pkg.dev/oss-exit-gate-prod/stitch-sdk--npm/`) + GCS release manifest upload (`gs://oss-exit-gate-prod-projects-bucket/stitch-sdk/npm/manifests/`).

### Key Changes
- **Keyless OIDC Authentication (`google-github-actions/auth@v3`)**: Authenticates via Workload Identity Pool `projects/305452601764/locations/global/workloadIdentityPools/builders/providers/github` (removing the need for long-lived publishing tokens).
- **Artifact Registry Staging**: Publishes release artifacts to the dedicated OSS Exit Gate staging registry (`https://us-npm.pkg.dev/oss-exit-gate-prod/stitch-sdk--npm/`).
- **Release Manifest Trigger (`google-github-actions/setup-gcloud@v3`)**: Uploads `manifest.json` (`{"publish_all": true}`) to `gs://oss-exit-gate-prod-projects-bucket/stitch-sdk/npm/manifests/` so OSS Exit Gate scans, signs/attests, and publishes the package upstream.

### Onboarding & Verification Context
- **Buganizer Tracking Issues:** `b/576708601`, `b/576673325`
- **Internal Onboarding CLs:** `cl/1004336531, cl/1004697825` (submitted; Smoke Test verified in OSS Exit Gate prod)
- **Maintainers / Recent Workflow Contributors:** cc @davideast