Add secret scanning alert notification workflow

[silvarohan18]

Summary

Adds an automated GitHub Actions workflow that monitors for secret scanning alerts and sends private Slack notifications to the h2o-ghas-alerts channel.

What it does

• Polls for new secret scanning alerts every 10 minutes via scheduled cron
• Sends detailed Slack notifications with: secret type, committer, commit SHA, file path, branch, and whether push protection was bypassed
• Tracks alert state via GitHub Actions cache to prevent duplicate notifications
• Includes a manual test mode (workflow_dispatch) to verify the integration without real alerts

Requirements

• SECRET_SCANNING_TOKEN — Fine-grained PAT with secret_scanning_alerts:read permission (org-level secret)
• H2O_OPS_SLACK_BOT_TOKEN — Slack bot token for posting to h2o-ghas-alerts channel (org-level secret)
• GitHub Advanced Security with secret scanning enabled on the repository

How to test

1. Go to Actions → "Secret Scanning Alert Notification"
2. Click "Run workflow" → check "Send mock test notification" → Run
3. Verify a test notification appears in #h2o-ghas-alerts