feat: Configurable signal handlers to stop in-progress deployments

[khvn26]

• Adds stopOnSignals configuration option to custom.deploymentSettings
• When the Serverless process receives a configured signal (e.g. SIGTERM, SIGINT), the plugin queries CodeDeploy for in-progress deployments and stops them with autoRollbackEnabled: true
• Enables cross-region rollback coordination when used with GitHub Actions fail-fast matrix strategy

Configuration

custom:
  deploymentSettings:
    stopOnSignals:
      - SIGTERM
      - SIGINT

How it works

1. Plugin registers handlers for each configured signal in the constructor
2. On signal receipt, it looks up the CodeDeploy application via CloudFormation
3. Lists all in-progress/queued/ready deployments for that application
4. Calls StopDeployment with auto-rollback on each (in parallel via Promise.all)
5. Errors are caught and logged — best-effort, doesn't block process exit

Test plan

• Unit tests for signal handler registration (configured, empty, missing)
• Unit tests for stopInProgressDeployments (stops deployments, no deployments, API errors)
• Integration test in staging: validate signal propagation with fail-fast: true and exec in CI