[zarturod]

Summary

Adds .kokoro/release.cfg and .kokoro/release.sh to migrate googleads-java-lib releases (Maven Central (com.google.api-ads:ads-lib*, dfp-*)) to OSS Exit Gate (googleads-java-lib/mavencentral) via the BAM DevRel Kokoro RBE release pool:

• Builds release artifacts in the Kokoro release container (ubuntu2204/full:current).
• Stages built artifacts to the internal OSS Exit Gate Artifact Registry staging repository (oss-exit-gate-prod).
• Supports DRY_RUN=true (default) to stage and validate artifacts without publishing, and DRY_RUN=false to upload manifest.json to gs://oss-exit-gate-prod-projects-bucket/googleads-java-lib/mavencentral/manifests to trigger automated BCID verification and production release.

Related Internal CLs

• Exit Gate Project Config (googleads-java-lib/mavencentral): http://cl/1004967549 (submitted) & http://cl/1008281146
• Kokoro Release Job Config (ads-api-devrel/bam-devrel/aim/googleads-java-lib-release.cfg): http://cl/1009767922

[zarturod]

PR closed, this is a major infrastructure change for repo owners
We shared the technical document with them to be reviewed, and afterward we will discuss and create a plan to work together.