feat: add new Bigtable feature

[danieljbruce]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request migrates system tests from Kokoro to Google Cloud Build for the handwritten/bigtable project. The existing Kokoro script is replaced with a notification, and a new cloudbuild.yaml is introduced. Feedback focuses on correcting the Cloud Build configuration, specifically addressing issues with environment variable persistence across steps, incorrect working directory paths, and the execution order of pre-test hooks. Consolidating steps and ensuring the correct directory context is used will prevent build failures and maintain coverage reporting.

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handwritten/bigtable/cloudbuild.yaml (6-55)

The current multi-step configuration has several issues that will likely cause the build to fail or behave incorrectly:

1. Environment Variable Persistence: Each Cloud Build step runs in a separate container. Environment variables exported by .kokoro/pre-system-test.sh in Step 2 will not be available to Step 3.
2. Working Directory: Cloud Build's default working directory is the repository root. Since this project is located in handwritten/bigtable, you must specify dir: 'handwritten/bigtable' for the steps to find package.json and the .kokoro directory. The current dir: '.' refers to the repo root, not the directory containing the cloudbuild.yaml file.
3. Execution Order: The original Kokoro script ran the pre-test hook before npm install. This is important if the hook configures registry credentials or environment-specific settings needed for installation.
4. Efficiency: npm install -g npm@latest and global prefix configuration are generally unnecessary in the Cloud Build environment and add overhead.

Consolidating these into a single step with the correct dir and execution order ensures correctness and improves performance.

- name: 'gcr.io/cloud-builders/npm'
  id: 'run-system-tests'
  entrypoint: 'bash'
  dir: 'handwritten/bigtable'
  args:
  - '-c'
  - |
    if [ -f .kokoro/pre-system-test.sh ]; then
      echo "Running pre-system-test.sh..."
      . .kokoro/pre-system-test.sh
    fi
    npm install
    npm run system-test
  env:
  - 'GCLOUD_PROJECT=${_GCP_PROJECT_ID}'

handwritten/bigtable/cloudbuild.yaml (58-77)

This step also requires the correct working directory (dir: 'handwritten/bigtable') to find the coverage artifacts. Additionally, the original Kokoro script uploaded coverage via codecov.sh, which is currently missing here. Consider adding the logic to upload reports to Codecov to avoid a regression in coverage tracking.

- name: 'gcr.io/cloud-builders/npm'
  id: 'coverage-report'
  entrypoint: 'bash'
  dir: 'handwritten/bigtable'
  args:
  - '-c'
  - |
    if [ -f ./node_modules/nyc/bin/nyc.js ]; then
      ./node_modules/nyc/bin/nyc.js report || true
    else
      echo "nyc not found, skipping coverage report."
    fi
    # TODO: Implement codecov upload logic here.
    echo "Codecov reporting (if desired) would be integrated here."