Add Slack alerting for scheduled job failures

[jwbron]

Summary:

This PR adds comprehensive Slack alerting functionality to the scheduled-job module. The alerting system uses Google Cloud Monitoring's native Slack webhook integration to send notifications when jobs fail.

Key features:

• Slack alerting enabled by default for all scheduled jobs
• Monitors Cloud Function failures, Cloud Run Job failures, and Cloud Scheduler failures
• Uses native Google Cloud Monitoring Slack webhook integration (no custom functions needed)
• Default channel set to #1s-and-0s
• Configurable webhook URL and channel
• Optional separate project for alerting resources
• Comprehensive monitoring policies for different failure scenarios

Issue: INFRA-10729

Test plan:

• Deploy the culture-cron with alerting enabled
• Verify monitoring policies are created in Google Cloud Console
• Test by simulating a job failure and confirming Slack notifications are received