Add CD workflow for Gradle Plugin Portal

[andrefigas]

Summary

• Adds a CD workflow that publishes the plugin to Gradle Plugin Portal on push to main
• Uses GitHub Environment release with required reviewer for secure access control over publish credentials

Manual setup required

1. Go to Settings > Environments > New environment > name it release
2. Protection rules: Required reviewers > add andrefigas
3. Deployment branches: Selected branches > main
4. Environment secrets: add GRADLE_PUBLISH_KEY and GRADLE_PUBLISH_SECRET

Test plan

• Complete manual setup above
• Merge a PR to main and verify the workflow triggers
• Approve the deployment and verify plugin appears on https://plugins.gradle.org/plugin/io.github.andrefigas.rustjni