[zarturod]

Summary

Adds .kokoro/release.cfg, .kokoro/release.sh, .kokoro/requirements.in, and .kokoro/requirements.txt to migrate googleads-python-lib PyPI releases (googleads) to the OSS Exit Gate pipeline via Kokoro.

• Build & Validate: Builds sdist and wheel distributions (python3 -m build) in an isolated Python 3.12 virtualenv and validates them with twine check.
• Stage to OSS Exit Gate Artifact Registry: Uploads distributions to https://us-python.pkg.dev/oss-exit-gate-prod/googleads-python-lib--pypi using ambient Kokoro BYOSA credentials (keyrings.google-artifactregistry-auth).
• Safe Dry-Run Default: Defaults to DRY_RUN=true in .kokoro/release.cfg and .kokoro/release.sh so staging can be verified before triggering external publication.
• Trigger Publication via Manifest: When DRY_RUN=false, uploads a targeted manifest.json ("name": "googleads") to gs://oss-exit-gate-prod-projects-bucket/googleads-python-lib/pypi/manifests/.
• Follows the BAM DevRel Kokoro + OSS Exit Gate pattern from googleads/data-manager-python/.kokoro/.

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